Post deadline Session on MgB₂

Talks 20 - 39

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Table of Contents
	Talks:   1-19     20-39     40-59     60-79
Talk 20	Lattice Properties of MgB2 versus Temperature and Pressure
Talk 21	The Dependence of Tc on Hydrostatic Pressure in Superconducting MgB2
Talk 22	Compressibility and Electronic Structure of MgB2 up to 8Gpa1
Talk 23	First principle pseudopotential calculation of the pressure dependence of the electronic properties of MgB2
Talk 24	Pressure Dependence of Electronic and Structural Properties of MgB2
Talk 25	Effect of hydrostatic pressure on the electronic properties and Tc in Superconducting MgB2
Talk 26	Thermodynamic and Transport Properties of Superconducting Mg10B2
Talk 27	Magnetoresistivity and Complete Hc2(T) in MgB2
Talk 28	Lower critical field at odds with a s-wave superconductivity in the new superconductor MgB2
Talk 29	Penetration Depth and Anisotropy in MgB2
Talk 30	Hole carrier in MgB2 characterised by Hall Measurements
Talk 31	Hall and resistivity measurements on MgB2 grown by two different methods
Talk 32	Microwave & rf properties of superconducting MgB2
Talk 33	Study of Superconductivity in MgB2
Talk 34	Calculation of Magnetic Penetration Depth in MgB2
Talk 35	Scanning Tunneling Spectroscopy in MgB2
Talk 36	Energy Gap from Tunneling and Metallic Sharvin Contacts in MgB2
Talk 37	Scanning Tunneling Spectroscopic Studies of the Pairing Potential and Pairing Symmetry of Highly Dense MgB2
Talk 38	Superconducting Gap of MgB2 Observed by High-Resolution Photoemission Spectroscopy
Talk 39	Normal and Superconducting Properties of MgB2 Bulk Samples.

 

Talk 20

Lattice Properties of MgB₂ versus Temperature and Pressure

J. D. Jorgensen, D. G. Hinks, and S. Short,

Materials Science Division, Argonne National Laboratory, Argonne, IL 60439.   

We have determined the structural properties of the superconducting compound MgB₂ as a function of temperature from 11 K to 297 K and as a function of hydrostatic pressure up to 0.62 GPa using neutron powder diffraction.  This compound, when compared to other diborides with the same structure, is characterized by unusually large anisotropies of both the thermal expansion and compressibility, with the c-axis responses being substantially larger in both cases.  We speculate that the comparatively weaker metal-boron bonding in MgB₂, manifest by these lattice responses, is important for establishing the structural features, most importantly a comparatively long c axis, that give rise to high-T_c superconductivity in this structure type.  A small positive change of the a-axis lattice parameter, and the absence of such a change along the c axis, at T_c is consistent with the importance of the B s bands and the B-B in-plane vibrational modes for  superconductivity in this compound.

 

Talk 21

The Dependence of T_c on Hydrostatic Pressure in Superconducting MgB₂

T. Tomita, J.J. Hamlin, and J.S. Schilling (Department of Physics, Washington University in St. Louis, MO), D.G. Hinks and J.D. Jorgensen (Materials Science Division, Argonne National Laboratory)

Like the isotope effect, the dependence of T_c on hydrostatic pressure can also give information on the nature of the superconducting pairing interaction. In simple-metal BCS superconductors, such as Al, Sn, In, and Pb, T_c is found to invariably decrease with hydrostatic pressure at a moderate rate due to lattice stiffening. Very recent structural studies[1] under pure hydrostatic (He-gas) pressure to 6 kbar reveal that the hexagonal structure of MgB₂ is considerably (64%) more compressible in the c than in the a direction. This implies that for MgB₂ particular care must be taken that the applied pressure is truely hydrostatic. We have recently determined T_c(P) for MgB₂ under pure hydrostatic (He-gas) pressure to 7 kbar. We compare our results to those of other groups and discuss the implications for the superconducting mechanism.

[1] J.D. Jorgensen, D.G. Hinks, S. Short, cond-mat/ and to be published.

 

Talk 22

Compressibility and Electronic Structure of MgB₂ up to 8Gpa¹

T.VOGT, G. SCHNEIDER, Physics Department, Brookhaven National Laboratory, Upton, NY 11973-5000 USA, J.A. HRILJAC, School of Chemistry, The University

of Birgmingham, Edgbaston, Birmingham B152TT, United Kingdom, G. YANG, J.S ABELL, School of Metallurgy and Materials, The University of Birmingham,

Edgbaston, Birmingham B152TT, United Kingdom

The lattice parameters of MgB₂ up to pressures of 8 Gpa was determined using high resolution x-ray powder diffraction in a diamond anvil cell. The bulk modulus, B₀, was determined to be 151 ± 5Gpa. Both experimental and first-principles calculations indicate nearly isotropic mechanical behavior under pressure. This small anisotropy is in contrast to the 2 dimensional nature of boron p states. The pressure dependence of the density of states at the Fermi level and a reasonable value for the average phonon frequency áwñ account within the context of BCS theory for the reduction of T_c under pressure.

 

Talk 23

First principle pseudopotential calculation of the pressure dependence of the electronic properties of MgB₂

HONG SUN*, DAVID ROUNDY, HYOUNG JOON CHOI, STEVEN G. LOUIE, MARVIN L. COHEN, Dept. of Physics, Univ. of California Berkeley and Lawrence Berkeley National Laboratory.

First principle pseudopotential calculations are carried out to determine the lattice structure, electron density of states and band structures of MgB₂ under hydrostatic pressures. The obtained results show only slight changes in the electron density of states and band structures of MgB₂ under pressures. The possibility to increase the T_c under hydrostatic and uniaxial tension is also discussed.

 

Talk 24

Pressure Dependence of Electronic and Structural Properties of MgB₂,

O. GULSEREN and T. YILDIRIM, NIST Center for Neutron Research, NIST, Gaithersburg, MD. 

We report first-principles calculations of pressure dependence of the electronic and structural properties of MgB₂. Compressibility, bulk modulus, and the change in the energies of the zone-center phonons with external pressure are calculated and compared with the available experimental data. We also estimated the pressure dependence of the electron-phonon coupling near the zone center using frozen phonon approach. In particular, we study the effect of the pressure on the recently discovered anharmonic in-plane boron modes[1]. These results are then used within the BCS theory to estimate the pressure dependence of the critical temperature T_{C .}

[1] “Giant Anharmonicity and Non-linear Electron-Phonon Coupling in MgB₂: First-principles calculations and Inelastic Neutron Scattering Study”,

T. Yildirim  et al (Submitted).

 

Talk 25

Effect of hydrostatic pressure on the electronic properties and T_c in Superconducting MgB₂

D. L. Novikov, Arthur D. Little, Inc, N. I. Mevedeva, Institute of Solid State Chemistry, Ekaterinburg, Russia, J. E . Medvedeva and A. J. Freeman, Northwestern University, Evanston, IL, J. D. Jorgensen, Argonne National Laboratory, Argonne, IL

The effect of high pressure on the electronic structure of the new medium-T_c superconductor  MgB₂ was investigated by the ab-initio FLMTO-GGA method. Electron-phonon coupling was calculated  within the "rigid muffin-tin approximation" and Tc was estimated using the McMillan formalism. We found that despite the decrease of the density of states at the Fermi level, the Hopfield constant increases with pressure. Thus, the main reason for the reduction of the electron-phonon coupling constant, and thus Tc, under pressure is due to the increase of phonon frequencies, which is sufficiently large to compensate the electronic effects. The energies of the Mg-Mg, Mg-B and B-B interatomic bonds were estimated and their role in the cohesive properties was established. The dependencies of these energies and electric field gradients at Mg and B atoms from hydrostatic pressure are also presented and discussed.

 

Talk 26

Thermodynamic and Transport Properties of Superconducting Mg¹⁰B₂.

D. K. Finnemore, J. E. Ostenson, S. L. Bud'ko, G. Lapertot, and P. C. Canfield, Ames Laboratory, U.S. Department of Energy and Dept. of Physics and Astronomy, Iowa State University.

Transport and thermodynamic properties of a sintered pellet of the newly discovered MgB₂ superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, H_c2, with magnetization data giving dH_c2/dT = 0.44 T/K at the transition temperature of T_c= 40.2 K. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of one Tesla. The value of dH_c/dT at T_c is estimated to be about 12 mT/K, a value similar to classical superconductors like Sn. Hence, the Ginsburg-Landau parameter k » 26. Estimates of the critical supercurrent density, J_c, using hysteresis loops and the Bean model give critical current densities on the order of 10⁵ A/cm². Hence the supercurrent coupling through the grain boundaries is comparable to intermetallics like Nb₃Sn.

 

Talk 27

Magnetoresistivity and Complete H_c2(T) in MgB₂.

M-H. Jung , NHMFL - Pulse Facility, Los Alamos, S. L. Bud'ko, C. Petrovic, G. Lapertot, C. E. Cunningham, P. C. Canfield Ames Laboratory and Dept. of Physics and Astronomy, Iowa State Univ., A. H. Lacerda, NHMFL - Pulse Facility, Los Alamos.

Detailed magneto-transport data on dense wires of MgB₂ are reported for applied magnetic fields up to 18 T. The temperature and field dependencies of the electrical resistivity are consistent with MgB₂ behaving like a simple metal and following a generalized form of Kohler's rule. In addition, given the generally higher T_c values and narrower resistive transition widths associated with MgB₂ synthesized in this manner, combined with applied magnetic fields of up to 18 T, an accurate and complete H_c2(T) curve could be determined. This curve agrees well with curves determined from lower field measurements on sintered pellets and wires of MgB₂. H_c2(T) is linear in T over a wide range of temperature (7 K < T < 32 K) and has an upward curvature for T close to T_c. These features are similar to other high k , clean limit, boron-bearing intermetallics: YNi₂B₂C and LuNi₂B₂C.

 

Talk 28

Lower critical field at odds with a s-wave superconductivity in the new superconductor MgB₂

S. L. Li, H. H. Wen*, Z. W. Zhao, Y. M. Ni, Z. A. Ren, G. C. Che, H. P. Yang, Z. Y. Liu and Z. X. Zhao. National Laboratory for Superconductivity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China

By using a superconducting interference device and a vibrating sample magnetometer, the lower critical field H_c1 has been carefully measured on a well shaped cylindrical sample of the new superconductor MgB₂ fabricated by high pressure synthesis. The penetration depth l is calculated from the H_c1(T) data. It is found that a linear relation of H_c1(T) appears in whole temperature region below T_c. Furthermore a finite slope of dH_c1(T)/dT and dl(T)/dT remains down to the lowest temperature ( 2 K ). These are at odds with the expectation for a widely thought s-wave superconductivity in MgB₂.

 

Talk 29

Penetration Depth and Anisotropy in MgB₂

X. H. Chen, Y. Y. Xue, R. L. Meng and C. W. Chu^*  TCSUH, University of Houston, Houston, TX 77204-5932 , * and LBL

The penetration depth l of MgB₂ was deduced from both the ac susceptibility c and the magnetization loop M(H) of sorted powders. The good agreement between the two sets of data without geometric correction for the grain orientation suggests that MgB₂ is an isotropic superconductor.

 

Talk 30

Superconducting Hole carrier in MgB₂ characterised by Hall Measurements

W. N. Kang, C. U. Jung, Kijoon H. P. Kim, Min-Seok Park, S. Y. Lee, Hyeong-Jin Kim, Eun-Mi Choi, Kyung Hee Kim, Mun-Seog Kim, 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

The longitudinal resistivity (r_xx) and Hall coefficient (R_H) were measured for MgB₂ sintered under high pressure. We found that R_H is positive like cuprate high-Tc superconductors, and decreases as temperature increases for 40 K < T< 300 K. The cotangent of Hall angle was found to follow a + bT² behavior from T_c to 300 K. At

T = 100 K, R_H = 4.1 x 10^-11 m³/C from which hole carrier density was determined to be 1.5 x 10²³ /cm³. This carrier density is 2 - 3 orders of magnitude larger than those of

Nb₃Sn and optimally doped YBa ₂Cu ₃O_y superconductors.

 

Talk 31

Hall and resistivity measurements on MgB₂ grown by two different methods

YAYU WANG, N.P.ONG, Department of Physics, Princeton University, Princeton, NJ 08544. N.ROGADO, K.A.REGAN, R.J.CAVA, Department of Chemistry, PrincetonUniversity, Princeton, NJ 08544

We report Hall and resistivity measurements on MgB₂ grown by two different methods. Sample 1 grown by high pressure sintering have a low RRR (1.3) and large resistivities (2m.cm at 300K). Sample 2 prepared in a Ta tube have larger RRR (9-10) and improved resistivities (60.cm at 300K). We have measured their Hall coefficients RH between T_c and 300K. RH is in general positive and weakly temperature dependent, but its temperature dependence is sensitive to the preparation method. The hole density is about 10²³/cm³. Values of H_irr and H_c2 inferred from flux-flow resistivity will be presented.

 

Talk 32

Microwave & rf properties of superconducting MgB₂

S.Sridhar^a, N.Hakim, P.V.Parimi, E.Nyeanchi, C.Kusko, Physics Department, Northeastern University, Boston, MA 02115, P.C.Canfield, S.L.Bud'ko and D.K.Finnemore, Ames Laboratory, U.S. Department of Energy and Dept. of Physics and Astronomy, Iowa State University

Measurements of the 10 GHz surface impedance Z_s = R_s - iX_s and the radio frequency penetration depth of dense MgB₂ wires and pellets are presented and discussed. Substantial improvements in R_s are observed upon improving the sample density and reducing porosity. The microwave absorption results are compared with measurements on low T_c (Nb) and high T_c (YBa₂Cu₃O_6.95) superconductors. The results are also compared with BCS calculations for an s-wave superconductor, which has been claimed to be the order parameter from other measurements. The rf penetration depth measurements provide information concerning vortex dynamics and Hc₁ and Hc₂. The results are discussed in the context of the nature of the superconducting state, intra- and inter-granular contributions, and the prospect for microwave applications.

^a e-mail: srinivas@neu.edu

 

Talk 33

Study of Superconductivity in MgB₂

M. H. BADR, ANJAN K. GUPTA, YURI SUSHKO, K.-W. NG, University of Kentucky

We have prepared bulk MgB₂ samples by heating mixture of Mg and B at 950^o C. Resistivity measurements showed superconducting transition with onset of 39.5 K and a transition width of 1K. SQUID measurements confirmed the sharpness of the transition and an unexpected large H_c1 of at least 0.1T was also observed. We have also performed preliminary study on this sample with a low temperature scanning tunneling microscope. We will show and discuss some of our results in this presentation.

 

Talk 34

Calculation of Magnetic Penetration Depth in MgB₂

Sang Boo Nam*, KRISS and In-Ho Lee, KIAS

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, we carried out quantitative calculations of the magnetic penetration depth length in all T ranges in the s-wave superconductor [Lee et al, cond-mat/0101011]. Here we compared our calculation with the linear T dependence of data in MgB2 [Li et al, cond-mat/0103032], and obtained a good agreement.  The linear T variation of superelectron density at low T does not imply the anisotropic order parameter.

*wonkinam@kriss.re.kr

 

Talk 35

Scanning Tunneling Spectroscopy in MgB₂

G. Karapetrov^a, M. Iavarone^a,b, W. K. Kwok^a, G. W. Crabtree^a, and D. G. Hinks^a

^a)Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439;

^b)INFM - Dipartimento di Scienze Fisiche of the University of Naples “Federico II” , Piazzale Tecchio 80, 80125 Naples, Italy

We present scanning tunneling microscopy measurements of the surface of superconducting MgB₂ with a critical temperature of 39K. The value of the superconducting gap is 5.2 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair-breaking as seen in the conductance spectra in fields up to 6 T.

 

Talk 36

Energy Gap from Tunneling and Metallic Sharvin Contacts in MgB₂

Herbert Schmidt, J.F. Zasadzinski, K.E. Gray and D.G. Hinks

Materials Sciences Division, Argonne National Laboratory, Argonne, IL, 60439

Point-contact tunnel junctions, using a Au tip on sintered MgB₂ pellets, reveal a sharp superconducting energy gap that is confirmed by subsequent metallic Sharvin contacts made on the same sample.  The peak in the tunneling conductance and the Sharvin contact conductance are closely fit by the BCS form for an s-wave superconductor with only thermal smearing.  The gap values of 4.3 meV are less than the weak-coupling BCS value of 5.9 meV for the bulk T_c of 39 K and this is possibly due to chemical degradation of the near-surface region.

 

Talk 37

Scanning Tunneling Spectroscopic Studies of the Pairing Potential and Pairing Symmetry of Highly Dense MgB₂

N.-C. YEH, C.-T. CHEN, P. SENEOR, Dept. of Physics, California Institute of Technology, Pasadena, CA; R. P. VASQUEZ, Jet Propulsion Laboratory, Pasadena, CA; C. U. JUNG, Min-Seok PARK, Heon-Jung KIM, Sung-Ik LEE, Pohang Univ. of Science and Technology, Pohang, Korea

Spatially resolved quasiparticle tunneling spectra on highly dense MgB₂ pellets with

T_c = 39 K are taken using a low-temperature STM. The spectra on the as-grown samples reveal long-range spatial homogeneity (> 400 nm) within each grain, and macroscopic scale variations at length scales beyond a few µm's. The (dI/dV)-vs.-V spectra are insensitive to the tunneling impedance, and are consistent with a pairing potential D ranging from 4.0 to 6.5 meV at 4.2 K. The universally pronounced V-shape spectra with vanishing density of states near the Fermi level do not agree well with BCS s-wave pairing, even if the effect of disorder is taken into account. Further BTK analysis based on the scenarios of anisotropic s-wave pairing and other pairing symmetries are considered and compared with the data. Tunneling spectra on etched MgB₂ samples that are free of surface carbonates and oxides according to the XPS studies are also taken and compared with those of as-grown MgB₂.

 

Talk 38

Superconducting Gap of MgB₂ Observed by High-Resolution Photoemission Spectroscopy

T. TAKAHASHI, T. SATO, S. SOUMA, Tohoku University, T. MURANAKA, J. AKIMITSU, Aoyama-Gakuin University

Ultrahigh-resolution photoemission measurements have been performed on MgB₂ polycrystals to study the superconducting gap and its symmetry. Temperature-dependent measurements above/below T_c (39K) shows an opening of a clear s-like superconducting gap at E_F, together with a sharp superconducting coherent peak located a little away from E_F at the superconducting state. No pseudogap-like behavior is observed above T_c. Numerical fittings to the photoemission spectra suggest that the

superconducting-gap value (D) is about 5 meV, which gives 2D /k_B T_c of about 3.

 

Talk 39

Normal and Superconducting Properties of MgB₂ Bulk Samples. (audio only, no slides)

T. MASUI^1,2, A. YAMAMOTO¹, K. YOSHIDA¹, H. UCHIYAMA¹, M. F. LIMONOV¹, S. LEE¹ and S. TAJIMA¹,

¹ Superconductivity Research Laboratory, ISTEC, Tokyo 135-0062, Japan,

² Japan Science and Technology Corporation Domestic Research Fellow

We prepared bulk samples of MgB₂ superconductor, purifying commercial powder by acid and sintering it under high pressure. The resistivity shows a typical metallic T-dependence with a sharp transition at 38K, which shifts towards low T with pressure (dT_c/dP=-1.8K/GPa), presumably owing to a decrease in the density of state at E_F. The Seebeck  and the Hall coefficients indicate that the carriers are holes in this system. The Fermi edge observed in the photoemission spectrum is sharp, comparable to that of gold, in contrast to the broad edges of high-T_c cuprates. The superconducting gap estimated from the edge shift is 2.4 ±2meV, which is much smaller than the gap of (La,Sr)₂CuO₄ with T_c=40K and rather close to the BSC-value. The Raman spectrum shows a couple of sharp peaks and two broad peaks centered at 210 cm^-1 and 630 cm^-1. Although some of them may originate from the phonons of the possible impurity phase (MgB₄), the observed phonon frequency range is higher than that (<600cm^-1) for the high-T_c cuprates.