Transcript Slide 1
Superconductors: Basic Concepts Daniel Shantsev AMCS group Department of Physics University of Oslo • History • Superconducting materials • Properties • Understanding • Applications Research School Seminar February 6, 2006 Discovery of Superconductivity Discovered by Kamerlingh Onnes in 1911 during first low temperature measurements to liquefy helium Whilst measuring the resistivity of “pure” Hg he noticed that the electrical resistance dropped to zero at 4.2K How small is zero? A lead ring carrying a current of several hundred ampères was kept cooled for a period of 2.5 years with no measurable change in the current 1913 The superconducting elements Li Be 0.026 Na K Transition temperatures (K) Critical magnetic fields at absolute zero (mT) Mg Ca Sc Ti 0.39 10 Rb Cs Sr Ba Y La 6.0 110 Zr V Cr Mn Fe Fe Co C N O F Ne Al Si P S Cl Ar Ge As Se Br Kr I Xe At Rn 1.14 10 Ni Cu Zn Ga 0.875 1.091 (iron) 5.3 5.1 Tc=1K Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te (at 20GPa) 9.5 0.92 7.77 0.51 0.03 0.56 3.4 3.72 5.38 142 Nb 0.546 (Niobium) 4.7 198 9.5 141 HfTc=9K Ta W Re Os Ir 1.4 20 0.655 16.5 0.14 1.9 0.12 B 4.483 0.012 0.1 c 83 H =0.2T 7 5 3 Pt Au 29.3 30 Hg Tl Pb 4.153 41 2.39 17 7.19 80 Bi Po Transition temperatures (K) and critical fields are generally low Metals with the highest conductivities are not superconductors The magnetic 3d elements are not superconducting ...or so we thought until 2001 Superconducting transition temperature (K) Superconductivity in alloys and oxides HgBa2Ca2Cu3O9 (under pressure) 160 Highest Tc 140 HgBa2Ca2Cu3O9 120 TlBaCaCuO 138 K (at normal pressure) BiCaSrCuO 100 YBa2Cu3O7 Liquid Nitrogen temperature (77K) 80 60 MgB2 (LaBa)CuO 40 20 1987 Hg Pb Nb 1910 NbC 1930 NbN Nb3Sn Nb3Ge V3Si 1950 1970 1990 General properties • Zero resistance at T<Tc (Kamerlingh Onnes, 1911) Ideal conductor (the resistive state is restored in a magnetic field or at high transport currents) • Magnetic field is excluded from a superconductor (Meissner & Ochsenfeld, 1933) Ideal diamagnet Superconductivity – Quantum phenomenon at macroscale Quantization of magnetic flux Deaver & Fairbank, 1961 B Long hollow cylinder 2 the magnetic flux through a superconducting ring is an integer multiple of a flux quantum 6 BCS Theory Bardeen Cooper Schriffer 1972 (1) Electrons combine in Cooper pairs due to interactions with phonons x (2) All Cooper pairs (bosons) condense into one quantum state separated by an energy gap from excited states Metal: many individual electrons Superconductor: all electrons move coherently Experimental evidence for BCS Ivar Giaver (UiO) direct experimental evidence of the existence of the energy gap 1973 From the Nobel lecture, http://nobelprize.org/physics/laureates/1973/ N S Superconductivity – Quantum phenomenon at macroscale Quantization of magnetic flux Deaver & Fairbank, 1961 B 2 BCS: All Cooper pairs are desribed by one wave function: =| | ei dx = 2 /0 = 2k B. Josephson Josephson effect S I 1973 What is the resistance of the junction? S For small currents, the junction is a superconductor! V I = Ic sin (1 - 2) Supercurrent Phase of the wave function Josephson interferometer Most sensitive magnetometer – SQUID (superconducting quantum interference device) SQUID sensitivity Heart fields Brains fields 10-14 T 10-10 T 10-13 T Magnetic field Hc Normal state Vortex lattice Type I A. A. Abrikosov Meissner state Temperature Hc2 Tc Normal state Mixed state (vortex matter) Hc1 2003 (published 1957) Type II Meissner state Temperature Tc Vortex normal core x Coherence length J x B(r) London penetration depth l l Flux quantum: B dA superconductor = h/2e = 0 l x<l type II NS interface x>l type I NS interface 12 Ginzburg-Landau Theory V. L. Ginzburg, L. D. Landau 2003 Order parameter? a T-Tc Ginzburg-Landau functional: 13 High-current Cables ~100 times better than Cu In May of 2001 some 150,000 residents of Copenhagen began receiving their electricity through high-Tc superconducting material (30 meters long cable). Magnetic Resonance Imaging (MRI) • 75 million MRI scans per year • Higher magnetic field means higher sensitivity Magnetoencephalography Measuring tiny magnetic fields in the human brain • Electric generators made with superconducting wire • Superconducting Magnetic Energy Storage System • Superconductor-based transformers and fault limiters • Infrared sensors • Magnetic shielding devices • Ultra-high-performance filters • etc Most high energy accelerators now use superconducting magnets. The proton accelerator at Fermilab uses 774 superconducting magnets (7 meter long tubular magnets which generate a field of 4.5 Tesla) in a ring of circumference 6.2 km. The coils are made of NbSn3 or NbTi embedded in form of fine filaments (20 mm diameter) in a copper matrix Image from BNL Superconducting magnet designed for the Alpha Magnetic Spectrometer at the International Space Station to help look for dark matter, missing matter & antimatter Image from U.Geneva Levitation: MagLev Trains Miyazaki Maglev Test Track, 40 km • No friction • Super-high speed • Safety • Noiseless 581 km/h Vortex pinning Record trapped field: 17 Tesla Field distribution Ba f J Jc Lorentz force: f = JB presintered 123-pellet • The maximal field in the magnets, • The maximal current in the cables are determined by vortex pinning => it’s important to study vortices Top-seeded melt-growth Superconductivity Lab @ UiO Magneto-optical imaging Åge Olsen: Observation of what Vortices do NbSe2 field-cooled to 4.3 K Sanyalak Niratisairak: Characterization of MO-films 10 mm Jørn Inge Vestgården: Calculation of Vortex distributions