Post
deadline Session on MgB2
Talks 40 - 59
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Slideshows containing audio and
slides are available for all linked talk titles. Not all talks are linked.
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Table of Contents
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| Talks: 1-19 20-39 40-59 60-79 | |
| Talk 40 | X-ray Photoemission Study of MgB2 |
| Talk 41 | Infrared properties of superconductor MgB2 |
| Talk 42 | Calculation of the Tunneling Conductance in MgB2 |
| Talk 43 | Specific Heat of Mg11B2 |
| Talk 44 | MgB2, A Conventional Superconductor With Strong Electron-Phonon |
| Talk 45 | Heat capacity measurements on MgB2. |
| Talk 46 | Superconductivity in Dense MgB2 Wires. |
| Talk 47 | Superconducting Properties of MgB2 Bulk Materials Prepared by High Pressure Sintering |
| Talk 48 | Loss of Superconductivity and Structural Transition in Mg1-xAlxB2. |
| Talk 49 | Effect of sintering temperature under high pressure in the superconductivity |
| Talk 50 | Superconducting transition of powdered and sintered MgB2 samples |
| Talk 51 | GGA Calculations of Diboride Structure, Stability and Density of States |
| Talk 52 | Electronic structure and superconductivity in MgB2 and related binary |
| Talk 53 | Thermodynamics of MgBx. |
| Talk 54 | Superconducting MgB2 films with Tc > 20K by Pulsed Laser Deposition. |
| Talk 55 | Synthesis and Properties of MgB2 Superconducting |
| Talk 56 | Superconducting MgB2 Thin Films. |
| Talk 57 | Strongly linked current flow in polycrystalline forms of the new superconductor |
| Talk 58 | Potential of MgB2 for Electric Power Applications. |
| Talk 59 | Prominent bulk pinning effect in the newly discovered MgB2 superconductor |
X-ray
Photoemission Study of MgB2
R.
P. VASQUEZ, Jet Propulsion Laboratory,
Pasadena, CA, C. U. JUNG, MIN-SEOK PARK, HEON-JUNG KIM, J. Y. KIM,
and SUNG-IK LEE, Pohang University
of Science and Technology, Pohang, Republic of Korea
A
high quality sintered pellet of MgB2 has been studied by
x-ray photoemission spectroscopy (XPS), and compared to measurements
from MgO and MgF2 single crystals. The as-grown surface has
a layer which is Mg-rich and appears to consist primarily of Mg(OH)2
and MgCO3, similar to observations on alkaline earth-containing
cuprate high temperature superconductor surfaces. The surface reacted
layer is effectively removed by etching, and the subsurface region is
Mg-deficient. This nonideal near-surface region may explain varied scanning
tunneling spectroscopy results. The XPS core level and Auger signals
of MgB2 are similar to measurements from metallic Mg and
transition metal diborides, and the measured valence band is consistent
with the calculated density of states.
Infrared
properties of superconductor MgB2
P.F.Mena1, A.B. Kuzmenko1,
D. van der Marel1*, B. Gorshunov2, C. Kuntscher2,
P. Haas2, M. Dressel2, T. Muranaka3,
J. Akimitsu3
1Laboratory of Solid State Physics, Materials Science Centre,
Nijenborgh 4, 9747 AG Groningen,The Netherlands.
*Presenting author
21. Physikalisches Institut, Universität
Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany
3Department of Physics, Aoyama-Gakuin University, 6-16-1 Chitsedai,
Setagaya-ku, Tokyo 157, Japan
Infrared reflection studies on the intermetallic
compound MgB2 which becomes superconducting at Tc=39
K were performed in the spectral range from 8 cm-1 to 10.000
cm-1 at temperatures 5 – 300 K, using normal incidence and
grazing incidence reflectometry. A clear signature of the superconducting
state is observed in the temperature dependence of the grazing-p-polarized
reflection spectra: The reflectivity increases abruptly below Tc, for
frequencies below 65 cm-1. This is an indication of increased
dielectric screening in the superconducting state. The upper frequency
of 65 cm-1 (2.4 kBTc)
of the reflectivity change may reflect a quasi-particle gap, or a Josephson
plasma resonance.
Calculation
of the Tunneling Conductance in MgB2
In-Ho Lee, KIAS and Sang Boo Nam*, KRISS
The notion of a finite pairing interaction
energy range Td [Nam, Phys. Lett. A193 (1994) 111; (E) A197 (1995) 458],
results in the incomplete condensation in which not all states are participate
in pairings. In the framework of a finite Td [Nam, J. Korean Phys. Soc.
28 (1995) 102], we present quantitative calculations of ratios of the
conductance in superconducting to that in normal states, in good agreements
with data of [Rubio-Bollinger et al, cond-mat/0102242] and [Krapetrov
et al, cond-mat/0102312], in the s-wave superconductor. We used new
density of states, not the modified BCS density of states with the depairing
parameter.
F. BOUQUET, R. A. FISHER, N. E. PHILLIPS,
Lawrence Berkeley National Laboratory
D. G. HINKS, J. D. JORGENSEN, Argonne
National Laboratory
The specific heat (C) of a polycrystalline sample of Mg11B2 has
been measured, 1-50 K, in magnetic fields (H)
to 9 T. There is a sharp specific-heat
anomaly at Tc =
38.7 K, DC
~ 116 mJ K-1
mol-1. The coefficient of the normal-state electron
contribution to C is
g
~ 2.9 mJ K-2
mol-1. Above 20 K the electron contribution to the
superconducting-state specific heat is approximately proportional to
T2; at lower temperatures there
is a feature that indicates structure in the gap. Extreme nonlinearity with H of the mixed-state g
may reflect both anisotropy
and gap structure.
MgB2,
A Conventional Superconductor With Strong Electron-Phonon
Ch. Waelti1, E. Felder1,
C. Degen1, G. Wigger1, R. Monnier, B. Delley2
and H.R. Ott1
1 Lab. f. Festkoerperphysik, ETH Zuerich, Switzerland; 2
PSI, Villigen, Switzerland.
Results of measurements of the specific
heat CP(T) of MgB2 between 3 and 220 K are reported.
A modified Debye-Einstein type model has been used to achieve an accurate
account of the lattice contribution to CP(T), allowing for
a subsequent evaluation of the electronic specific heat CPel
above and below the transition temperature Tc. From our analysis
and taking into account published values for the electronic density
of states at the Fermi energy N(EF), we obtain an electron-phonon
coupling parameter l ≈
2, significantly enhanced above
the common weak-coupling values l
≤ 0.4. The CPel
(T) data can be accounted for by a conventional BCS approximation, taking
into account particular features of the lattice excitation spectrum.
Considering a non negligible difference between Cp and CV
at elevated temperatures, the parameter l may be smaller, but still exceed the value
of 1.
Heat
capacity measurements on MgB2.
S. Patnaik, L.D. Cooley, M. Rikel, A.A.
Polyanskii, J.Y. Jiang, X.Y. Cai, A. Gurevich, C.B. Eom, E.E. Hellstrom,
and D.C. Larbalestier, Applied Superconductivity Center, University of Wisconsin–Madison, S.L.
Bud’ko, C. Petrovic, G. Lapertot, C.E. Cunningham, and P.C. Canfield,
Ames National Laboratory.
We characterized the new superconductor
MgB2 using the heat capacity measurement. From the zero magnetic
field and infield data up to 1T, we determined the transition temperature
and the bulk upper critical field and made a comparison with the values
determined from the magnetization measurements. Our results are based
on samples prepared using three different processing techniques.
Superconductivity
in Dense MgB2 Wires.
P. C. Canfield, D. K. Finnemore, S. L. Bud'ko, J. E.
Ostenson, G. Lapertot, C. E. Cunningham, and C. Petrovic, Ames Laboratory, U.S. Department of Energy and Dept. of Physics and Astronomy,
Iowa State University.
MgB2
becomes superconducting just below 40 K. Whereas porous polycrystalline
samples of MgB2 can be synthesized from boron powders, we
demonstrate that dense wires of MgB2 can be prepared by exposing
boron filaments to Mg vapor. The resulting wires have a diameter of
160 mm, are better
than 80% dense and manifest the full c
= -1/4p
shielding in the superconducting state. Temperature-dependent resistivity
measurements indicate that MgB2 is a highly conducting metal
in the normal state with r(40
K) = 0.38 mOhm-cm.
Using this value, an electronic mean free path, l »
600 Å can be estimated, indicating that MgB2 wires are well
within the clean limit. Tc, Hc2(T), and Jc
data indicate that MgB2 manifests comparable or better superconducting
properties in dense wire form than it manifests as a sintered pellet.
Superconducting Properties of MgB2 Bulk Materials Prepared by High Pressure Sintering (no audio, slides only)
Y. Takano, H. Takeya, H. Fujii , H. Kumakura,
T. Hatano and K. Togano
National
Research Institute for Metals, 1-2-1, Sengen, Tsukuba 305-0047, Japan.
CREST,
Japan Science and Technology Corporation, 2-1-6, Sengen, Tsukuba 305-0047,
Japan
H. Kito and H. Ihara
Electrotechnical
Laboratory, 1-1-4, Umezono, Tsukuba 305-8568, Japan
High-density bulk materials of a newly discovered
40K intermetallic MgB2 superconductor were prepared by high pressure sintering. Superconducting transition with the onset temperature
of 39K was confirmed by both magnetic and resistive measurements. Magnetization
versus field (M-H) curve shows the behavior of a typical Type II superconductor
and the lower critical field Hc1(0) estimated from M-H curve
is 0.032T. The bulk sample shows
good connection between grains and critical current density Jc
estimated from the magnetization hysteresis using sample size was 2x104A/cm2
at 20K and 1T. Upper critical
field Hc2(0) determined by extrapolating the onset of resistive
transition and assuming a dirty limit is 18T
Loss
of Superconductivity and Structural Transition in Mg1-xAlxB2.
J.S. Slusky, N.
Rogado, K.A. Regan, M.A. Hayward, P. Khalifah, T. He, K. Inumaru, S.M.
Loureiro, M.K. Haas, H.W. Zandbergen and R.J. Cava, Department of Chemistry and Princeton
Materials Institute, Princeton University, Princeton NJ 08544
It is shown that the addition of electrons
to MgB2 through partial substitution of Al for Mg results
in the loss of superconductivity. For Al contents less than 10% the
transition temperature decreases only slightly. Associated with larger
amounts of Al substitution is a subtle but distinct structural transition.
The primary characteristic of the transition is the partial collapse
of the spacing between boron layers near 10% Al content. Bulk superconductivity
disappears at the same Al concentration. This indicates that superconducting
MgB2 is poised very near a structural instability at slightly
higher electron concentrations.
Effect
of sintering temperature under high pressure in the superconductivity
for MgB2
C.
U. Jung, Min-Seok Park, W. N. Kang, Mun-Seog Kim, Kijoon H. P. Kim,
S. Y. Lee, and Sung-Ik Lee.
National Creative Research Initiative Center for Superconductivity and Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
We
report the effect of the sintering temperature on the superconductivity
of MgB2 pellets prepared under a high pressure of 3 GPa.
The superconducting properties of the non-heated MgB2 in
this high pressure were poor. However, as the sintering temperature
increased, the superconducting properties were vastly enhanced, which
was shown by the narrow transition width for the resistivity and the
low-field magnetizations. The observed surface morphology using scanning
electron microscope showed that these changes were closely related to
changes in the microscopic level such as the connectivity of the grains.
High resolution transmission electron microscope images showed also
that the grains were tightly connected without interfacial impurity
layers after sintering under high pressure.
Superconducting
transition of powdered and sintered MgB2 samples,
H. CLAUS, U. WELP, V. K. VLASKO--VLASOV,
D. G. HINKS, and G. W. Crabtree, Materials Science Division, Argonne
National Laboratory, Argonne, IL 60439.
We have determined the diamagnetic transition
of sintered and powdered MgB2 samples in fields between 0.05
and 30 G. The powdered samples show a sharp transition at 39.2 K which
is only slightly depressed with increasing magnetic field. However,
the superconducting transition of the sintered sample is broad and strongly
field-dependent. At the lowest fields, a two step transition is revealed.
We demonstrate that the two steps correspond to the intra-grain and
inter-grain superconducting transition.
GGA
Calculations of Diboride Structure, Stability and Density of States
John E. Jaffe, Pacific Northwest National Laboratory
I have performed total energy calculations
with plane waves, ultrasoft pseudopotentials, the generalized gradient
approximation (GGA) and conjugate gradient geometry optimization for
BeB2, MgB2, CaB2, SrB2,
BaB2, ZnB2, CdB2, CuB2,
NaB2 and AlB2. By subtracting the total energies of the respective
pure metals I determine a relative heat of formation of all the other
compounds with respect to MgB2. I find that only BeB2, CaB2, SrB2
and AlB2 are likely to be stable in the same structure as
MgB2, among the compounds studied.
By assuming that the parameters entering the BCS-McMillan Tc
formula are similar for all these compounds except for the coupling
constant l, which scales with the computed density
of states N(0) at the Fermi energy, I predict that AlB2 and
BeB2 have zero or low Tc’s, while CaB2
has Tc ~ 50K and SrB2 may have an even higher
transition temperature
Electronic
structure and superconductivity in MgB2 and related binary
and ternary borides.
N. I. Mevedeva, A. L. Ivanovskii,
Institute of Solid State Chemistry,
Ekaterinburg, Russia, J. E. Medvedeva and A. J. Freeman, Northwestern University, Evanston, IL
The electronic structure of
(i) the new medium-Tc superconductor MgB2, (ii) other stable and hypothetical diborides MB2 (M=Be,
Al,Ca, Sc, Y), and (iii) nonstoichiometric
and ternary MgB2-based diborides were investigated using
the ab-initio FLMTO-GGA approach.
Together with estimates of the electron-phonon coupling and Tc,
the results obtained indicate that superconductivity in MgB2
can be attributed to the existence of px,y-band holes at
G, explain the absence of superconductivity for BeB2, AlB2,
ScB2 and YB2 and suggest that CaB2
could be the closest analogue of MgB2, as expected. The simulation
of ternary systems Mg1-xMx B2 (M =
Be, Ca, Li, Na), MgB2-yXy (X = C, N, O) and nonstoichiometric
Mgx<1B2 and MgBy<2 demonstrate
that electron doping of MgB2 acts unfavorably and that a
possible way of searching for higher Tc may be via additional
hole doping of MgB2 (or CaB2) or via creating layered superstructures
of the MgB2/CaB2 type. Comparing results for Li
and Na substitutions and Mgx<1B2, we found
that N(EF) is independent of hole doping level and dopant
and the energy of the top px,y band at G relative to EF increases linearly with hole concentration.
ZI-KUI LIU, D. G. SCHLOM, Dept of MS&E,
Penn State University, QI LI, X. X. XI, Dept of Physics, Penn State
University.
We have studied thermodynamics of MgBx
with a thermodynamic modeling technique, CALPHAD, using a computerized
optimization procedure. Temperature-composition, pressure-composition,
and pressure-temperature phase diagrams under different conditions are
obtained. We find that the superconducting phase, MgB2, is
favored with above-stoichiometric Mg content and in a pressure-temperature
window. The implications of these results on the processing of MgB2
thin films with various deposition techniques are discussed.
Superconducting
MgB2 films with Tc > 20K by Pulsed Laser Deposition.
H.Y. Zhai, C. Cantoni,
M. Paranthaman, B. C. Sales, C. Rouleau, D.K. Christen, D.H. Lowndes,
and H.M. Christen, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831-6056.
Superconducting films of MgB2
were prepared by Pulsed Laser Deposition on various metal and oxide
substrates. Four-probe resistivity measurements reveal a Tc
(zero resistance) of 22K with an onset at 28K. Results from the resistivity
data above Tc and from x-ray diffraction indicate that these
films are multiphase and presumably Mg-deficient. We present results
from different approaches to control the Mg:B ratio and discuss the
influence of growth temperature and other deposition parameters on the
stoichiometry and structure of these materials.
Synthesis
and Properties of MgB2 Superconducting
Thin Films
C.B. Eom† § , M.K. Lee†,
J.H. Choi† §, L. Belenky†, S. Patnaik§,
A.A. Polyanskii§, E.E
Hellstrom† §, D. C. Larbalestier† §, † Department
of Materials Science and Engineering, §Applied Superconductivity
Center, University of Wisconsin-Madison, Madison, WI 53706; N. Rogado, K.A. Regan, M.A. Hayward, T. He, J.S. Slusky, K. Inumaru,
M.K.
Haas and R.J. Cava, Department of Chemistry
and Princeton Materials Institute, Princeton University, Princeton,
NJ 08544
We report the synthesis and properties of
MgB2 superconducting thin films grown by pulsed laser deposition.
Magnetization measurement exhibits the superconducting transition
temperature of 30K. An atomic force microscope image of the film
shows hexagonal shaped grains, which is consistent with the crystal
symmetry of MgB2 on the basal plane. We will discuss the
structural and superconducting properties of the films.
Superconducting
MgB2 Thin Films.
S. H. MOON, J. H. YUN, J. I.
KYE, H. G. KIM, and B. OH, LG Electronics
Institute
of Technology,
Seoul 137-724, Korea.
Superconducting MgB2 thin films were prepared on several different substrates including Al2O3
and silicon wafer.
MgB2 thin films were deposited by the electron-beam evaporation. Various substrate temperatures during deposition
and proper annealing conditions of films were investigated. Superconducting
transition temperature (Tc) and critical current density
(Jc) of the films were measured. The surface morphology was
examined by scanning electron microscope (SEM) and atomic force microscope
(AFM). X-ray diffraction data of thin films showed the MgB2 phase in random orientation. Thin films
of Tc above 20 K and Jc of about 105
A/cm2 at 4.2 K have been obtained, and the progress on the
higher Tc and Jc is underway.
Strongly
linked current flow in polycrystalline forms of the new superconductor
MgB2
D.C. LARBALESTIER, M. RIKEL, L.D. COOLEY,
A.A. POLYANSKII, J.Y. JIANG, S. PATNAIK, X.Y. CAI, D.M. FELDMANN, A.
GUREVICH, A.A. SQUITIERI, M.T. NAUS, C. B. EOM AND E.E. HELLSTROM, Applied Superconductivity Center, University of Wisconsin–Madison, 1500
Engineering Drive, Madison, WI 53706, R.J. CAVA , K.A. REGAN, N.
ROGADO, M.A. HAYWARD, T. HE, J.S. SLUSKY, P. KHALIFAH, K. INUMARU, AND
M. HAAS, Department of Chemistry and Princeton Materials
Institute, Princeton University, Princeton NJ 08544
Polycrystalline samples of MgB2
were analyzed to determine whether grain boundaries are weak links to
current flow, as in high-temperature superconductors.
VSM and SQUID magnetometry from 4-45 K and 0-14 T show large
hysteresis and temperature scaling of the flux pinning force consistent
with conventional flux-pinning behavior with average, whole-sample current
density above 104 Acm-2 at 20 K, 1 T.
Hc2(4.2K) is 16-17 T and the field at which
Jc goes to zero
using standard Kramer function scaling is ~7 T (4.2K), suggestive of
weakened regions of superconductivity within the samples. Magneto-optical (MO) analyses show position-sensitive bulk flux
gradients which vary locally by at least two orders of magnitude, the
best regions on scales of ~100 mm exhibiting ~105 A/cm2
(11K, 0.1T). Analytical scanning
electron microscopy shows that the strong superconducting regions consist
of a sponge-like network of ~100 nm grains, pores, and boron-rich phases
that may have formed by partial in situ decomposition of the pre-formed
MgB2. Both across the matrix and within the locally
high Jc regions,
strong grain-to-grain connections are inferred, contrary to the situation
for polycrystalline high temperature superconductors where weak coupled
grain boundaries are a major issue.
Paper to appear in Nature, March 8, 2001
Potential
of MgB2 for Electric Power Applications.
P. M. GRANT, Electric Power Research Institute. Palo Alto, CA 94304.
Notwithstanding the fact that the now-known
existence of superconductivity below 40 K in the intermetallic compound
MgB2 is barely a few months old, sufficient data has already
emerged to allow a preliminary assessment of its potential for electric
power equipment and infrastructure application. Even at this early stage of investigation, present values of technical
parameters such as critical current density, intergrain coupling and
irreversibility field in a projected operation range of 25 – 30 K are
already of encouraging magnitude and will only increase as vortex pinning
mechanisms are understood and enhanced in what is now a relatively “clean”
material. Moreover, there are signs that practical wire
development is a distinct possibility at reasonable cost given the huge
natural abundance of its constituent elements and widespread experience
in commercial metallurgical manufacturing of similar materials. Thus superconductivity in MgB2 opens
a technical window to a range of electric power applications previously
thought accessible only with copper oxide perovskites. These include superconducting rotating machinery,
transformers, magnetic energy storage, and cables. In this talk, we will address the extent of
this opportunity, and establish preliminary targets of performance and
cost prospective MgB2 wire would have to meet to fully capture
its promise.
Prominent
bulk pinning effect in the newly discovered MgB2 superconductor
Mun-Seog
Kim, C. U. Jung, Min-Seok Park, S. Y. Lee, Kijoon H. P. Kim, W. N. Kang,
and Sung-Ik Lee.
National Creative Research Initiative Center for Superconductivity and
Department of Physics, Pohang University of Science and Technology,
Pohang 790-784, Republic of Korea
We
report the magnetic-field dependence of the irreversible magnetization
of the recently discovered binary superconductor MgB2. For
the temperature region of T < 0.9 Tc , the contribution
of the bulk pinning to the magnetization overwhelms that of the surface
pinning. This was evident from the fact that the magnetization curves,
M ( H ), were well described by the critical-state model without considering
the surface pinning effect. It was also found that the M ( H ) curves
at various temperatures scaled when the field and the magnetization
were normalized by the characteristic scaling factors H* ( T ) and M*
( T ), respectively. This feature suggests that the pinning mechanism
determining the hysteresis in M ( H ) is unique below T = Tc
.