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Paramagnetism and reentrant behavior in quasi-one-dimensional superconductors at high magnetic fields

Description: The thermodynamics of quasi-one-dimensional superconductors in the presence of large magnetic fields is studied. When the quantum effects of the magnetic field are taken into account, several reentrant superconducting phases persist at very high fields. In the last reentrant phase the free energy change, the specific heat jump and the excess magnetization are estimated near the critical temperature. In particular, the excess magnetization is found to be paramagnetic as opposed to diamagnetic in weak fields and its sign is controlled by the slope of H{sub c{sub 2}} (T). The authors further generalize this result to the entire phase diagram (including all quantum phases) and to different physical systems using general thermodynamic relations which show that the sign of the excess magnetization {Delta}M of the superconducting state near H{sub c{sub 2}}(T) follows dH{sub c{sub 2}}(T)/dT. These relations provide a scenario for the evolution of the sign of {Delta}M from weak fields to strong fields.
Date: February 1996
Creator: Sa de Melo, C. A. R.
Partner: UNT Libraries Government Documents Department

Limits of NbTi and Nb3Sn, and development of W& R Bi-2212 High Field Accelerator Magnets

Description: NbTi accelerator dipoles are limited to magnetic fields (H) of about 10 T, due to an intrinsic upper critical field (H{sub c2}) limitation of 14 T. To surpass this restriction, prototype Nb{sub 3}Sn magnets are being developed which have reached 16 T. We show that Nb{sub 3}Sn dipole technology is practically limited to 17 to 18 T due to insufficient high field pinning, and intrinsically to 20 to 22 T due to H{sub c2} limitations. Therefore, to obtain magnetic fields approaching 20 T and higher, a material is required with a higher H{sub c2} and sufficient high field pinning capacity. A realistic candidate for this purpose is Bi-2212, which is available in round wires and sufficient lengths for the fabrication of coils based on Rutherford-type cables. We initiated a program to develop the required technology to construct accelerator magnets from 'wind-and-react' (W&R) Bi-2212 coils. We outline the complications that arise through the use of Bi-2212, describe the development paths to address these issues, and conclude with the design of W&R Bi-2212 sub-scale magnets.
Date: June 1, 2007
Creator: Cheng, Daniel; Dietderich, Daniel; Ferrracin, Paolo; Prestemon, Soren; Sabbi, GianLuca; Scanlan, Ron et al.
Partner: UNT Libraries Government Documents Department

Intrinsic Spin-Orbit Coupling in Superconducting Delta-Doped SrTiO3 Heterostructures

Description: We report the violation of the Pauli limit due to intrinsic spin-orbit coupling in SrTiO{sub 3} heterostructures. Via selective doping down to a few nanometers, a two-dimensional superconductor is formed, geometrically suppressing orbital pair-breaking. The spin-orbit scattering is exposed by the robust in-plane superconducting upper critical field, exceeding the Pauli limit by a factor of 4. Transport scattering times several orders of magnitude higher than for conventional thin film superconductors enables a new regime to be entered, where spin-orbit coupling effects arise non-perturbatively.
Date: August 19, 2011
Creator: Bell, Christopher
Partner: UNT Libraries Government Documents Department

Superconductivity in SrNi2P2 single crystals

Description: Heat capacity, magnetic susceptibility, and resistivity of SrNi{sub 2}P{sub 2} single crystals are presented, illustrating the structural transition at 325 K, and bulk superconductivity at 1.4 K. The magnitude of {Tc}, fits to the heat capacity data, the small upper critical field H{sub c2} = 390 Oe, and {kappa} = 2.1 suggests a conventional fully gapped superconductor. With applied pressure we find that superconductivity persists into the so-called 'collapsed tetragonal' phase, although the transition temperature is monotonically suppressed with increasing pressure. This argues that reduced dimensionality can be a mechanism for increasing the transition temperatures of layered NiP, as well as layered FeAs and NiAs, superconductors.
Date: January 1, 2009
Creator: Ronning, Filip; Bauer, Eric D; Park, Tuscon & Thompson, Joe D
Partner: UNT Libraries Government Documents Department

Vortices in dense self-assembled hole arrays.

Description: We present a study of the upper critical field and pinning strength from the resistivity and magnetization of a Nb film containing a dense array of 45 nm diameter holes on a hexagonal lattice with a spacing of 101 nm. The holes were formed by self-assembly in anodic aluminum oxide (AAO) using an electrochemical procedure. Confinement effects and Little-Parks oscillations are seen above 6 K, and strong pinning with matching field effects is seen below 6 K. Above the first matching field interstitial vortices coexist with vortices trapped in the hole array. Pinning in the Nb films with hole arrays is enhanced by two orders of magnitude over that in continuous Nb films. At low temperature, flux avalanches are observed and imaged using the magneto-optical Faraday effect.
Date: October 9, 2002
Creator: Crabtree, G. W.; Welp, U.; Xiao, Z. L.; Jiang, J. S.; Vlasko-Vlasov, V. K.; Bader, S. D. et al.
Partner: UNT Libraries Government Documents Department

Effect of defects on the critical points in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: The upper and lower critical points are investigated in untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals with dilute columnar defects. Dilute columnar defects raise the upper critical point, indicating that the transition near the upper critical point is a vortex entanglement transition. The lower critical point is very sensitive to columnar defect disorder and its position can be described by a Lindemann-like criterion similar to that for melting. Dilute columnar defects induce non-linear behavior in the I-V curves of the vortex liquid state above the lower critical point, which the authors interpret as a vestige of the critical region associated with the Bose glass transition below the lower critical point.
Date: January 26, 2000
Creator: Kwok, W. K.; Olsson, R. J.; Karapetrov, G.; Paulis, L. M.; Moulton, W. G.; Hofman, D. J. et al.
Partner: UNT Libraries Government Documents Department

Kinetics of phase growth in Nb3Sn formation for heat treatment optimization

Description: The kinetics of growth and superconducting properties of Nb{sub 3}Sn are investigated as a function of the heat treatment (HT) duration and temperature for Internal Tin and Powder-in-Tube strands at 650, 700 and 750 C. For all times and temperatures, the Nb{sub 3}Sn layer thickness is measured, the critical current at 4.2 K is tested as a function of magnetic field, and the upper critical field is evaluated. Results of the layer critical current density are also shown as a function of HT duration and temperature.
Date: October 25, 2002
Creator: Barzi, Emanuela & Mattafirri, Sara
Partner: UNT Libraries Government Documents Department

Landau quantization and superconductivity at high magnetic fields

Description: Recently, the possibility of superconducting for fields stronger than the semiclassical H{sub c2} due to Landau quantization has been raised again. In this paper, the theory of such a state is reviewed for both 2D and 3D systems. We explicitly discuss the effects of impurities and Zeeman splitting. A critical analysis of the possibility of observing this reentrant behavior in real materials is given. Finally, solutions of the mean field gap equations (Bogoliubov-deGennes equations) in the quantum limit are presented and discussed in regards to their flux lattice structure.
Date: May 1, 1991
Creator: Norman, M. R.; Akera, H. & MacDonald, A. H.
Partner: UNT Libraries Government Documents Department


Description: We have measured the critical magnetic field for superconductivity In tungsten from 5.5 to 15 mK using a {gamma}-ray anisotropy thermometer, and we have measured the heat capacity between 0.35 and 25 K. Analysis of the data gives H{sub 0} = 1.237 Oe for the 0 K critical field, T{sub c} = 16.0 mK for the critical temperature, {gamma} = 1.008 mJ/mole K{sup 2} for the coefficient of the electronic heat capacity, and {Theta}{sub 0} = 383 K for the 0 K Debye temperature. The measured values of the critical field H{sub c} are consistently higher than those reported by Black, Johnson and Wheatley (BJW) on the CMN temperature scale, but the temperature dependence is similar. This discrepancy and the temperature dependence of H{sub c} suggest that both sets of H{sub c} data are affected by magnetic impurities. Use of the calorimetric {gamma} value permits an improved test of the CMN temperature scale with the very low temperature H{sub c} data obtained by BJW.
Date: March 1, 1973
Creator: Triplett, B.B.; Phillips, N.E.; Thorp, T.L.; Shirley, D.A. & Brewer, W.D.
Partner: UNT Libraries Government Documents Department

2010 POLYMER PHYSICS - JUNE 27 - JULY 2, 2010

Description: The 2010 Gordon Research Conference on Polymer Physics will provide outstanding lectures and discussions in this critical field that impacts every industrial sector from electronics to transportation to medicine to textiles to energy generation and storage. Fundamental topics range from mechanical properties of soft gels to new understanding in polymer crystallization to energy conversion and transmission to simulating polymer dynamics at the nanoscale. This international conference will feature 22 invited lectures, wherein the opening 10 minutes are specifically designed for a general polymer physics audience. In addition, poster sessions and informal activities provide ample opportunity to discuss the latest advances in polymer physics. The technical content of the meeting will include new twists on traditional polymer physics topics, recent advances in previously underrepresented topics, and emerging technologies enabled by polymer physics. Here is a partially listing of targeted topics: (1) electrically-active and light-responsive polymers and polymer-based materials used in energy conversion and storage; (2) polymers with hierarchical structures including supramolecular assemblies, ion-containing polymers, and self-assembled block polymers; (3) mechanical and rheological properties of soft materials, such as hydrogels, and of heterogeneous materials, particularly microphase separated polymers and polymer nanocomposites; and (4) crystallization of polymers in dilute solutions, polymer melts, and miscible polymer blends.
Date: July 2, 2010
Creator: Winey, Karen
Partner: UNT Libraries Government Documents Department

Bulk superconductivity in PuCoGa[sub 5] at 18.5[sup +]

Description: Elemental plutonium is a fascinating metal yet poorly understood. A significant aspect of its complexity is the ambivalent nature of Pu's 5f electrons, which frequently appear to be neither completely localized nor hlly itinerant. This issue is emphasized in PuCoGas, the first Pu-based superconductor and with a T, exceeding 18 K. Though conventional phonon-mediated pairing may be responsible for its superconductivity, this view must be reconciled with three observations: (1) above T,, the static susceptibility of PuCoGa5 is Curie-Weiss-like with an effective moment of 0.68 p {approx}c,lo se to that expected for Pu'; (2) its Sommerfeld specific heat coefficient of 77 mJ/moleK2 indicates that electronic correlations cannot be ignored; and, (3) isostructural UCoGa5 is a Pauli paramagnet with much smaller Sommerfeld coefficient and is not superconducting above 0.3 K. Rather than a conventional superconductor, PuCoGa5 appears to be more nearly analogous to the isostructural heavy-fmion superconductor CeCoIn5 in which antiferromagnetic spin fluctuations mediate Cooper pairing. Also as in CeCoIn5, the T=O orbital upper critical field of 74 T in PuCoGas exceeds the Pauli limit by nearly a factor of two. In a scenario of magnetically mediated superconductivity, the almost order of magnitude higher T, in PuCoGas would be attributed to stronger 5f-ligand hybridization. Perhaps PuCoGas bridges two classes of unconventional superconductors-the heavy-fermion systems and high-Tc cuprates.
Date: January 1, 2002
Creator: Thompson, J. D. (Joe David)
Partner: UNT Libraries Government Documents Department

Advanced materials, strands, and conductors for particle accelerators. Technical report for the year 1994

Description: The authors research for the Division of High Energy Physics (HEP) began with studies of both the superconducting and matrix components of multifilamentary composites, viz (1) attempts to increase the flux-pinning strength in NbTi, and (2) a method of suppressing proximity effect coupling in fine-filament strands. The latter was fully successful, and stands ready to be invoked as soon as the need arises to: (a) very closely space the filaments in the interests of quality, or (b) re-introduce fine-filament composites for strand-magnetization reduction or AC-loss minimization. But there were also many spin-offs during the life of the program, as indicated in the complete list of publications (copy available on request). For instance, the various other effects and properties that were studied and published over the period of this association with HEP include: (i) reduction in T{sub c} due to proximity effect between thin {alpha}phase precipitates and the NbTi matrix, (ii) critical field enhancement with reduction of filament diameter in fine-filament composites, (iii) studies and systematics of AC loss in composite strands, (iv) compensation of strand magnetization by means of Ni plating or filament substitution, (v) hysteretic loss due to surface pinning in multifilamentary NbTi, (vi) flux creep in SSC-type strands, (vii) temperature and field dependence of eddy current decay, (vii) influence of Mn doping on the properties of NbTi, (viii) transverse and longitudinal resistivities (including a size-effect contribution) in strands with various filament/matrix configurations.
Date: September 6, 1994
Partner: UNT Libraries Government Documents Department

Superconductivity in Strong Magnetic Field (Greater Than Upper Critical Field)

Description: The National High Magnetic Field Laboratory, funded by the National Science Foundation and other US federal Agencies, has in recent years built a wide range of magnetic fields, DC 25 to 35 Tesla, short pulse 50 - 60 Tesla, and quasi-continuous 60 Tesla. Future plans are to push the frontiers to 45 Tesla DC and 70 to 100 Tesla pulse. This user facility, is open for national and international users, and creates an excellent tool for materials research (metals, semiconductors, superconductors, biological systems ..., etc). Here we present results of a systematic study of the upper critical field of a novel superconducting material which is considered a promising candidate for the search for superconductivity beyond H{sub c2} as proposed by several new theories. These theories predict that superconductors with low carrier density can reenter the superconducting phase beyond the conventional upper critical field H{sub c2}. This negates the conventional thinking that superconductivity and magnetic fields are antagonistic.
Date: August 22, 1998
Creator: Tessema, G.X.; Gamble, B.K.; Skove, M.J.; Lacerda, A.H. & Mielke, C.H.
Partner: UNT Libraries Government Documents Department

The Dirac Experiments - Results and Challenges

Description: The 1997 international Dirac II Series held at Los Alamos National Laboratory involved low temperature electrical transport and optical experiments in magnetic fields exceeding 800%, produced by explosive flux compression using Russian MC-1 generators. An overview of the scientific and technical advances achieved in this Series is given, together with a strategy for future work in this challenging experimental environment. A significant outcome was achieved in transport studies of microfabricated thin-film YBCO structures with the magnetic field in the CuO plane. Using a GHz transmission line technique at an ambient temperature of 1.6 K, an onset of dissipation was observed at 150 T (a new upper bound for superconductivity in any material), with a saturation of resistivity at 240 T. Comparison with the Pauli limit expected at B=155 T in this material suggests that the critical field in this geometry is limited by spin paramagnetism. In preparation for a Diract III series, a systematic temperature-dependent transport study of YBCO using in-plane magnetic fields of 150 T generated by single-turn coils, at temperatures over the range 10-100 K, has been undertaken in collaboration with the Japanese Megagauss Laboratory. The objective is to map out the phase diagram for this geometry, which is expected to be significantly different than the Werthamer-Helfand-Hohenberg model, due to the presence of paramagnetic limiting. Nanofabricated magnetometers have also been developed in a UNSW-LANL collaboration for use in Dirac III for Fermi surface measurements of YBCO in megagauss fields, which are described.
Date: October 24, 1998
Creator: Clark, R.G.; O'Brien, J.L.; Dzurak, A.S.; Kane, B.E.; Lumpkin, N.E.; Reilley, D.J. et al.
Partner: UNT Libraries Government Documents Department

Reentrant softening as precursor to reentrant melting of the vortex-lattice in YBCO single crystal

Description: A vibrating sample technique was used to study the elastic behavior of the magnetic vortex system in YBa{sub 2}Cu{sub 3}O{sub 7} single crystal. The setup consists of a system of two weakly coupled mechanical oscillators (transducer, sample), the frequency and Q of which depends sensitively on the frequencies of the two subsystems as well as the coupling between both. By sweeping a magnetic field at temperatures below the superconducting transition temperature {Tc} the authors observe pronounced attenuation peaks of temperature-dependent characteristic field strengths H{sub 1} and H{sub 2}. These fields mark temperature-dependent points of constant elasticity of the vortex-ensemble. Since softening precedes the melting of the vortex-lattice by approaching H{sub o1} as well as H{sub o2}, the observed angular dependence of H{sub 1} and H{sub 2} is interpreted as due to reentrant softening as precursor to reentrant melting.
Date: October 12, 1999
Creator: Hucho, C.; Carter, J. M.; Muller, V.; Petrean, A. & Kwok, W. K.
Partner: UNT Libraries Government Documents Department

Characteristic features of the exotic superconductors: A summary

Description: The authors summarize the results of a comprehensive examination of the characteristic features of the exotic superconductors, the superconductors so-labelled by Uemura and co-workers. In both the electronic and the crystal-chemistry properties, they find anomalous features which appear to be universal for these materials, as well as other features which are clearly not universal but common enough to be considered typical for these materials. Some implications of these anomalies are discussed.
Date: September 1, 1997
Creator: Brandow, B.
Partner: UNT Libraries Government Documents Department

Phase diagram of single crystal MgB{sub 2}.

Description: Using magnetization, magneto-transport and single-crystal specific heat measurements the authors have determined the superconducting phase diagram of MgB{sub 2}. The superconducting anisotropy {gamma} changes monotonously from a value of around 2 near T{sub c} to above 4.5 at 22 K. For H{parallel}c a pronounced peak effect in the critical current occurs at the upper critical field. They present a strong evidence for a surface superconducting state for H{parallel}c which might account for the wide spread in reported values of the superconducting anisotropy {gamma}.
Date: September 30, 2002
Creator: Karapetrov, G.; Welp, U.; Rydh, A.; Iavarone, M.; Kwok, W. K.; Crabtree, G. W. et al.
Partner: UNT Libraries Government Documents Department

Magnetic measurements of the lower critical fields and irreversible properties of oxygen deficient YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} single crystals

Description: We present results for lower critical field, irreversibility field and hysteresis on an oxygen deficient single crystal of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} annealed to a {Tc} of 28 and 85 K. We compare results for off-plateau ({Tc} = 28 and {Tc} = 85 K) stoichiometries to earlier results on the 60 K {Tc} plateau crystals and the fully oxygenated 92 K {Tc} crystals. We find a systematic decrease in the lower critical field with decreasing {Tc}, and a decrease in the remanent magnetization by a factor of 10, at T = 5 K, for the {Tc} = 28 K phase compared to the {Tc} = 85 K phase. Our irreversibility field versus reduced temperature data indicate much stronger curvature and hence a larger reversible region for the reduced oxygen stoichiometries. For crystals with {Tc} = 62.2, 56, 50, and 28 K, a crossover occurs in the slope of log(H{sub irr}) vs log(l-T/{Tc}). The field at which this crossover occurs is lower for the more oxygen deficient phases and may be linked to their lower H{sub c1} values and corresponding longer penetration depths.
Date: February 1, 1992
Creator: Vandervoort, K. G.; Welp, U.; Claus, H.; Crabtree, G. W. & Fang, Y.
Partner: UNT Libraries Government Documents Department

Complex microwave conductivity of YBa{sub 2}Cu{sub 3}O{sub 7} in magnetic fields up to 500T

Description: We have measured the complex conductivity of thin films of YBa{sub 2}cu{sub 3}O{sub 7} (YBCO) superconductor down to temperatures of 4K and magnetic fields up to 500T. The highly oriented films were probed by 94 GHz radiation, with the external magnetic field applied perpendicular to the c-axis. These measurements allowed us to assign a value of 340 {plus_minus} 40 for the upper critical field at T=O. The measurements were recently carried out at the pulsed field facility of the National High Magnetic Field Laboratory at Los Alamos using both Russian and American magnetic flux compression generators.
Date: July 1, 1994
Creator: Goettee, J. D.; Zerwekh, W. D. & Kudasov, Y. B.
Partner: UNT Libraries Government Documents Department

Radiation effects on superconductivity

Description: The effect of radiation on the superconducting transition temperature (T/ sub c/), upper critical field (H/sub c2/), and volume-pinning-force density (F/ sub p/) were discussed for the three kinds of superconducting material (elements, alloys, and compounds). 11 figures, 3 tables, 86 references. (auth)
Date: January 1, 1975
Creator: Brown, B.S.
Partner: UNT Libraries Government Documents Department

Stripe-to-bubble transition of magnetic domains at the spin reorientation of (Fe/Ni)/Cu/Ni/Cu(001)

Description: Magnetic domain evolution at the spin reorientation transition (SRT) of (Fe/Ni)/Cu/Ni/Cu(001) is investigated using photoemission electron microscopy. While the (Fe/Ni) layer exhibits the SRT, the interlayer coupling of the perpendicularly magnetized Ni layer to the (Fe/Ni) layer serves as a virtual perpendicular magnetic field exerted on the (Fe/Ni) layer. We find that the perpendicular virtual magnetic field breaks the up-down symmetry of the (Fe/Ni) stripe domains to induce a net magnetization in the normal direction of the film. Moreover, as the virtual magnetic field increases to exceed a critical field, the stripe domain phase evolves into a bubble domain phase. Although the critical field depends on the Fe film thickness, we show that the area fraction of the minority domain exhibits a universal value that determines the stripe-to-bubble phase transition.
Date: June 9, 2010
Creator: Wu, J.; Choi, J.; Won, C.; Wu, Y. Z.; Scholl, A.; Doran, A. et al.
Partner: UNT Libraries Government Documents Department

A general scaling relation for the critical current density inNb3Sn

Description: We review the scaling relations for the critical currentdensity (Jc) in Nb3Sn wires and include recent findings on the variationof the upper critical field (Hc2) with temperature (T) and A15composition. Measurements of Hc2(T) in inevitably inhomogeneous wires, aswell as analysis of literature results, have shown that all availableHc2(T) data can be accurately described by a single relation from themicroscopic theory. This relation also holds for inhomogeneity averaged,effective, Hc2*(T) results and can be approximated by Hc2(t)=Hc2(0) =1-t1.52, with t = T=Tc.Knowing Hc2*(T) implies that also Jc(T) is known.We highlight deficiencies in the Summers/Ekin relations, which are notable to account for the correct Jc(T) dependence. Available Jc(H) resultsindicate that the magnetic field dependence for all wires from mu0H = 1 Tup to about 80 percent of the maximum Hc2 can be described with Kramer'sflux shear model, if non-linearities in Kramer plots when approaching themaximum Hc2 are attributed to A15 inhomogeneities. The strain (e)dependence is introduced through a temperature and strain dependentHc2*(T,e) and Ginzburg-Landau parameter kappa1(T,e) and a straindependent critical temperature Tc(e). This is more consistent than theusual Ekin unification of strain and temperature dependence, which usestwo separate and different dependencies on Hc2*(T) and Hc2*(e). Using acorrect temperature dependence and accounting for the A15 inhomogeneitiesleads to the remarkable simple relation Jc(H,T,e)=(C/mu0H)s(e)(1-t1.52)(1-t2)h0.5(1-h)2, where C is a constant, s(e)represents the normalized strain dependence of Hc2*(0) andh =H/Hc2*(T,e). Finally, a new relation for s(e) is proposed, which is anasymmetric version of our earlier deviatoric strain model and based onthe first, second and third strain invariants. The new scaling relationsolves a number of much debated issues withrespect to Jc scaling in Nb3Snand is therefore of importance to the applied community, who use scalingrelations to analyze magnet performance from wire results.
Date: May 8, 2006
Creator: Godeke, A.; Haken, B. ten; Kate, H.H.J. ten & Larbalestier, D.C.
Partner: UNT Libraries Government Documents Department