DOCSLIB.ORG

Superconductivity: the Meissner Effect, Persistent Currents, and The

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Superconductivity: the Meissner Effect, Persistent Currents, and The Superconductivity: The Meissner Effect, Persistent Currents, and the Josephson Effects MIT Department of Physics (Dated: March 10, 2005) Several of the phenomena of superconductivity are observed in three experiments carried out in a liquid helium cryostat. The transition to the superconducting state of each of several bulk samples of Type I and II superconductors is observed in measurements of the exclusion of magnetic field (the Meisner effect) from samples as the temperature is gradually reduced by the flow of cold gas from boiling helium. The persistence of a current induced in a superconducting cylinder of lead is demonstrated by measurements of its magnetic field over a period of a day. The tunneling of Cooper pairs through an insulating junction between two superconductors (the DC Josephson effect) is demonstrated, and the magnitude of the fluxoid is measured by observation of the effect of a magnetic field on the Josephson current. 1. PREPARATORY QUESTIONS temperature. The liquid helium (liquefied at MIT in the Cryogenic Engineering Laboratory) is stored in a highly 1.How would you determine whether or notinsulated a sample Dewar flask. Certain precautions must be taken of some unknown material is capable ofin being its a manipulation.It is imperative that you read superconductor? the following section on Cryogenic Safety before proceeding with the experiment. 2.Calculate the loss rate of liquid helium from the dewar in L−1. d The density of liquid helium at 4.2K is 0.123− g3. cm At STP helium gas has a density of 0.178−1. g Data L for the measured2.1. CRYOGENIC ASPECTS AND SAFETY boil­off rates are given later in the labguide. The Dewar flask shown in Figure 5 contains the liq­ 3.Consider two solenoids, each of length 3uid cm, helium di­ in a nearly spherical metal container (34 cm ameter 0.3 cm, and each consisting of 200in turns diameter) of which is supported from the top by a long copper wire wrapped uniformly and on topaccess of one or neck tube (length about 50­60 cm and diame­ another on the same cylindrical form. A solidter cylin­ about 3 cm) of low conductivity metal.∼25 It holds der of lead of length 3 cm and diameterliters 0.2 of cm liquid. is The boiling rate of the liquid helium is located inside the common interior volumedetermined of the by the rate of heat transfer which is slowed solenoids. Suppose there is an alternating currentby a surrounding vacuum space which minimizes direct with an amplitude of 40 mA and a frequencythermal of conduction 5 and multiple layers of aluminized my­ KHz in one of the solenoids. What is thelar induced which minimizes radiative coupling from the room. open circuit voltage across the second solenoidThe be­ internal surfaces are all carefully polished and silver fore and after the lead is cooled belowplated its critical for low radiation emissivity. Measurement shows temperature? that helium gas from the boiling liquid in a quiet Dewar 4.What is the Josephson effect? is released with a flow rate of gas at STP (standard tem­ perature and press ure = 760 torr and 293K) about 5 5.Calculate the critical current for a 1mm diametercm3s−1. Niobium wire at liquid Helium temperatures (4.2The only access to the helium is through the neck of K). Use equation 1 to evaluate the criticalthe magnetic Dewar. Blockage of this neck by a gas­tight plug will field at 4.2 K and knowing that Bo =result 0.2 Tesla in a for build­up of pressure in the vessel and a con­ Niobium. sequent danger of explosion. The most likely cause of a neck blockage is frozen air (remember everything except helium freezes hard at 4 K) in lower sections of the neck 2. INTRODUCTION tube. Thus it is imperative to inhibit the streaming of air down the neck. In normal quiet storage condition, In this experiment you will study several ofthe the top re­ of the neck is closed with a metal plug resting markable phenomena of superconductivity, a propertyloosely on the top flange. When the pressure in the De­ that certain materials (e.g. lead, tin, mercury)war exhibit rises above0= p W/A (where w is the weight of the when cooled to very low temperature. As theplug cooling and A is its cross­sectional area) the plug rises and agent you will be using liquid helium which boilssome at pressure 4.2 is released. Thus the pressure in the closed K at standard atmospheric pressure. With suitableDewar op­ is regulated0 atwhich p has been set at approx­ eration of the equipment you will be able toimately control 0.5 the in. Hg.This permits the vaporized helium temperatures of samples in the range above thisgas boiling to escape and prevents a counterflow of air into the Id: 39.superconductivity.tex,v 1.106 2005/02/22 20:00:56 sewell Exp Id: 39.superconductivity.tex,v 1.106 2005/02/22 20:00:56 sewell Exp2 neck. When the plug is removed, air flows downstream into the neck where it freezes solid. You will have to re­ move the plug for measurements of the helium level and for inserting probes for the experiment,it is and impor­ tant that the duration of this open condition be minimized. Always have the neck plug inserted when the Dewar is not in use. In spite of precautionary measures, some frozen air will often be found on the surface of the neck. This can be scraped from the tube surface with the neck reamer, con­ sisting of a thin­wall half­tube of brass. Insertion of the warm tube will liquefy and evaporate the oxygen ­ ni­ trogen layer or knock it into the liquid helium where it will be innocuous.If you feel any resistance while the probe is being inserted in the Dewar, remove the probe immediately and ream the surface (the whole way around) before reinserting the probe. The penalty for not doing this may be stickingFIG. of 1: deli­ Critical field plotted against temperature for various cate probe components to the neck surface. Warmedsuperconductors. and refrozen air is a strong, quick­drying glue! In summary: Zero resistivity is, of course, an essential characteris­ 1.Always have the neck plug on the Dewartic when of it a is superconductor. However superconductors ex­ not in use. hibit other properties that distinguish them from what one might imagine to be simply a perfect conductor, i.e. 2.Minimize the time of open­neck condition whenan ideal in­ substance whose only peculiar property is zero serting things into the Dewar. resistivity. The temperature below which a sample is supercon­ 3.Ream the neck surface of frozen sludge if the probe ducting in the absence of a magnetic field is called the doesn’t go in easily. critical temperature,Tc. At any given temperature, T < Tc, there is a certain minimumBc(T ), field called the critical field, which will just quench the superconduc­ 3. THEORETICAL BACKGROUND tivity. It is found (experimentally and theoretically) that Bcis relatedT toby the equation 3.1. Superconductivity � �T �2� Superconductivity was discovered in 1911 by H. Bc=B01 − (1) Kamerlingh Onnes in Holland while studying the electri­ Tc cal resistance of a sample of frozen mercury as a function whereB is the asymptotic value of the critical field of temperature. On cooling with liquid helium, which he0 asT → 0 K. Figure 1 shows this dependence for various was the first to liquify in 1908, he found that the resis­ materials including those you will be studying in this tance vanished abruptly at approximately 4 K. In 1913 experiment. he won the Nobel prize for the liquification of helium Note the extreme range of the critical field values. and the discovery of superconductivity. Since that time, These diagrams can be thought of as phase diagrams: be­ many other materials have been found to exhibit this low its curve, a material is in the superconducting phase; phenomenon. Today over a thousand materials, includ­ above its curve the material is in the non­conducting ing some thirty of the pure chemical elements, are known phase. to become superconductors at various temperatures rang­ ing up to about 20 K. Within the last several years a new family of ceramic compounds has been discovered which are insulators at room temperature and superconductors3.2. Meissner Effect and the Distinction Between a at temperatures of liquid nitrogen. Thus, far fromPerfect be­ Conductor and a Super Conductor ing a rare physical phenomenon, superconductivity is a fairly common property of materials. Exploitation ofAn this electric fieldE� in a normal conductor causes a cur­ resistance­free property is of great technical importancerent densityJ� which, in a steady state, is related to the in a wide range of applications such as magneticelectric reso­ field by the equationJ� =σE� whereσ is called the nance imaging and the bending magnets in particleelectrical ac­ conductivity. In a metal the charge carriers are celerators such as the Tevatron at Fermi Lab.electrons, and at a constant temperatureσ is a constant Id: 39.superconductivity.tex,v 1.106 2005/02/22 20:00:56 sewell Exp3 characteristic of the metal. In these commonmedium circum­ attained its perfect conductivity, after which it stances the relation betweenE�andJ � is called “Ohm’scannot be changed. It came as quite a surprise when law”. A material in whichσis constant is called an ohmicMeissner and Ochsenfeld in 1933 showed by experiment conductor. that this was not true for the case of superconductors. Electrical resistance in an ohmic conductor isThey caused found instead that the magnetic field inside a su­ by scattering of the individual conduction electronsperconductor by is always zero,B� = i.e.
Load more

Recommended publications
  • Lecture Notes: BCS Theory of Superconductivity
    Lecture Notes: BCS theory of superconductivity Prof. Rafael M. Fernandes Here we will discuss a new ground state of the interacting electron gas: the superconducting state. In this macroscopic quantum state, the electrons form coherent bound states called Cooper pairs, which dramatically change the macroscopic properties of the system, giving rise to perfect conductivity and perfect diamagnetism. We will mostly focus on conventional superconductors, where the Cooper pairs originate from a small attractive electron-electron interaction mediated by phonons. However, in the so- called unconventional superconductors - a topic of intense research in current solid state physics - the pairing can originate even from purely repulsive interactions. 1 Phenomenology Superconductivity was discovered by Kamerlingh-Onnes in 1911, when he was studying the transport properties of Hg (mercury) at low temperatures. He found that below the liquifying temperature of helium, at around 4:2 K, the resistivity of Hg would suddenly drop to zero. Although at the time there was not a well established model for the low-temperature behavior of transport in metals, the result was quite surprising, as the expectations were that the resistivity would either go to zero or diverge at T = 0, but not vanish at a finite temperature. In a metal the resistivity at low temperatures has a constant contribution from impurity scattering, a T 2 contribution from electron-electron scattering, and a T 5 contribution from phonon scattering. Thus, the vanishing of the resistivity at low temperatures is a clear indication of a new ground state. Another key property of the superconductor was discovered in 1933 by Meissner.
    [Show full text]
  • Superconductivity: the Meissner Effect, Persistent Currents and the Josephson Effects
    Superconductivity: The Meissner Effect, Persistent Currents and the Josephson Effects MIT Department of Physics (Dated: March 1, 2019) Several phenomena associated with superconductivity are observed in three experiments carried out in a liquid helium cryostat. The transition to the superconducting state of several bulk samples of Type I and II superconductors is observed in measurements of the exclusion of magnetic field (the Meisner effect) as the temperature is gradually reduced by the flow of cold gas from boiling helium. The persistence of a current induced in a superconducting cylinder of lead is demonstrated by measurements of its magnetic field over a period of a day. The tunneling of Cooper pairs through an insulating junction between two superconductors (the DC Josephson effect) is demonstrated, and the magnitude of the fluxoid is measured by observation of the effect of a magnetic field on the Josephson current. 1. PREPARATORY QUESTIONS 0.5 inches of Hg. This permits the vaporized helium gas to escape and prevents a counterflow of air into the neck. Please visit the Superconductivity chapter on the 8.14x When the plug is removed, air flows downstream into the website at mitx.mit.edu to review the background ma- neck where it freezes solid. You will have to remove the terial for this experiment. Answer all questions found in plug for measurements of the helium level and for in- the chapter. Work out the solutions in your laboratory serting probes for the experiment, and it is important notebook; submit your answers on the web site. that the duration of this open condition be mini- mized.
    [Show full text]
  • A Classical Deviation of the Meissner Effect in a Classical Textbook
    A Purely Classical Derivation of the Meissner Effect? A.M. Gulian∗ (Advanced Physics Laboratory, Chapman University, 15202 Dino Dr., Burtonsville, MD 20861, USA) Abstract.— A recent study (arXiv:1109.1968v2, to be published in American Journal of Physics) claims to provide a classical explanation of the Meissner effect. However, the argument misuses de Gennes’ derivation of flux expulsion in superconductors. In a recent publication [1], Essén and Fiolhais attempt to explain the Meissner effect in superconductors in a “purely classical” way. They utilize many arguments, most of which we will ignore in this short remark. Rather, we address their most crucial argument based on the excerpts from de Gennes’ classical textbook [2]. Following de Gennes, Ref. 1 writes the supercurrent density as j(r) = n(r)ev(r) , (1) where n is the density of superconducting electrons. In Ref. 1, they label v as the electron velocity, whereas De Gennes explicitly declares it to be carriers drift1 velocity. Substituting this equation into the expression for kinetic energy 1 E = n(r)mv 2 (r)dV , (2) k ∫ 2 subject to ∇ × h = (4π / c) j , and minimizing the sum of kinetic and magnetic energy E = (h2 /8π )dV mag ∫ with respect to the configuration of local magnetic field h, one can arrive at the F. and H. Londons’ equation: Submitted 1/29/2012 h + λ2∇ × (∇ × h) = 0 (3) where λ is the penetration depth. Equation (3), of course, explains the field repulsion. Based on this, and also noticing that the derivation of Eq. (3) “utilizes no quantum concepts and contains no Planck constant,” Essén and Fiolhais eventually deduce that superconductors “are just perfect conductors.” De Gennes himself never drew this conclusion.
    [Show full text]
  • The Physics and Applications of Superconducting Metamaterials
    The Physics and Applications of Superconducting Metamaterials Steven M. Anlage1,2 1Center for Nanophysics and Advanced Materials, Physics Department, University of Maryland, College Park, MD 20742-4111 2Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742 We summarize progress in the development and application of metamaterial structures utilizing superconducting elements. After a brief review of the salient features of superconductivity, the advantages of superconducting metamaterials over their normal metal counterparts are discussed. We then present the unique electromagnetic properties of superconductors and discuss their use in both proposed and demonstrated metamaterial structures. Finally we discuss novel applications enabled by superconducting metamaterials, and then mention a few possible directions for future research. speculates about future directions for these Our objective is to give a basic metamaterials. introduction to the emerging field of superconducting metamaterials. The I. Superconductivity discussion will focus on the RF, microwave, Superconductivity is characterized by and low-THz frequency range, because only three hallmark properties, these being zero DC there can the unique properties of resistance, a fully diamagnetic Meissner effect, superconductors be utilized. Superconductors and macroscopic quantum phenomena.1,2,3 have a number of electromagnetic properties The zero DC resistance hallmark was first not shared by normal metals, and these discovered by Kamerlingh Onnes in 1911, and properties can be exploited to make nearly has since led to many important applications ideal and novel metamaterial structures. In of superconductors in power transmission and section I. we begin with a brief overview of energy storage. The second hallmark is a the properties of superconductors that are of spontaneous and essentially complete relevance to this discussion.
    [Show full text]
  • 14.4. the Ginzburg–Landau Theory the BCS Theory Answered the Question Why Electrons Pair Up
    Phys520.nb 119 This is indeed what one observes experimentally for convectional superconductors. 14.3.7. Experimental evidence of the BCS theory III: isotope effect Because the attraction is mediated by phonons in the BCS theory, the transition temperature should depend on the mass of nucleons. As shown above, for the BCS theory, Tc ∝ ϵD, where ϵD is the Debye energy. For an isotropic elastic medium, it is 6 π2 13 ϵD = ℏωD = ℏ v (14.9) VC -1/2 Here, VC is the volume of a unit cell and v is the sound velocity. The sound velocity is typically proportional to M , where M is the mass of the nucleons. For example, in Chapter 3, we calculated before the sound velocity for a 1D crystal, which has K v = a (14.10) M Here, a is the lattice spacing, K is the spring constant of the bond and M is the mass of the atom. -1/2 Therefore, we found that Tc ∝ ϵD ∝ v ∝ M , so the BCS theory predicts that α = 1/2 in the isotope effect, which is indeed what observed in experiments. 14.3.8. Experimental evidence of the BCS theory IV (the direct evidence): charge are carried by particles with charge -2 e, instead of -e Q: How to measure the charge of the carriers? A: the Aharonov–Bohm effect. Aharonove and Bohm told us that if we move a charged particle around a closed loop, the quantum wave function will pick up a phase Δϕ Δϕ = q ΦB /ℏ (14.11) where q is the charge of the particle and ΦB is the magnetic flux → → → → ΦB = B·ⅆ S = A·ⅆ r (14.12) S We know that in quantum physics, particles are waves and thus they have interference phenomenon.
    [Show full text]
  • Arxiv:2104.02218V1 [Cond-Mat.Quant-Gas] 6 Apr 2021
    Persistent currents in rings of ultracold fermionic atoms Yanping Cai, Daniel G. Allman, Parth Sabharwal, and Kevin C. Wright∗ Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover NH 03766, USA We have produced persistent currents of ultracold fermionic atoms trapped in a toroidal geometry with lifetimes greater than 10 seconds in the strongly-interacting limit. These currents remain stable well into the BCS limit at sufficiently low temperature. We drive a circulating BCS superfluid into the normal phase and back by changing the interaction strength and find that the probability for quantized superflow to reappear is remarkably insensitive to the time spent in the normal phase and the minimum interaction strength. After ruling out the Kibble-Zurek mechanism for our experi- mental conditions, we argue that the reappearance of superflow is due to long-lived normal currents and the Hess-Fairbank effect. Coherent quantum systems can support currents that remain constant in time without being driven by any ex- ternal power source. These remarkable \persistent" cur- rents may occur as metastable non-equilibrium states in superconducting [1] and superfluid [2] phases, but equi- librium persistent currents also occur in normal conduct- ing phases around closed paths shorter than the coher- ence length [3{5]. Progress in understanding transport phenomena in quantum fluids has often been made by considering spherical, cylindrical, toroidal, or more exotic geometries [6], and realizing experimentally viable quan- tum many-body systems in more exotic geometries is an important challenge in quantum engineering. Persistent FIG. 1. Column density distribution for an equal spin mixture currents have been observed in experiments with Bose- of 1:2(1)×104 6Li atoms in a ring-dimple trap, which changes Einstein condensates (BECs) of ultracold atoms [7{9], when the interactions are tuned from the (a) BEC limit to the quantized phase slips [10] have been observed in matter- (d) BCS limit using a Feshbach resonance at 83.2 mT.
    [Show full text]
  • Superconducting Shielding W.O
    Superconducting shielding W.O. Hamilton To cite this version: W.O. Hamilton. Superconducting shielding. Revue de Physique Appliquée, Société française de physique / EDP, 1970, 5 (1), pp.41-48. 10.1051/rphysap:019700050104100. jpa-00243373 HAL Id: jpa-00243373 https://hal.archives-ouvertes.fr/jpa-00243373 Submitted on 1 Jan 1970 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. REVUE DE PHYSIQUE APPLIQUÉE: TOME 5, FÉVRIER 1970, PAGE 41. SUPERCONDUCTING SHIELDING By W. O. HAMILTON, Stanford University, Department of Physics, Stanford, California (U.S.A.). Abstract. 2014 Superconducting shields offer the possibility of obtaining truly zero magnetic fields due to the phenomenon of flux quantization. They also offer excellent shielding from external time varying fields. Various techniques of superconducting shielding will be surveyed and recent results discussed. 1. Introduction. - Superconductivity has been a field of active research interest since its discovery in 1911. Many of the possible uses of superconducti- vity have been apparent from that time but it has been only recently that science and technology have progressed to the point that it has become absolutely necessary to use superconductors for important and practical measurements which are central to disciplines other than low temperature physics.
    [Show full text]
  • Stabilization of Phenomenon and Meaning
    European Journal for Philosophy of Science manuscript No. (will be inserted by the editor) Stabilization of Phenomenon and Meaning On the London & London episode as a historical case in philosophy of science Jan Potters Received: date / Accepted: date Abstract In recent years, the use of historical cases in philosophy of science has become a proper topic of reflection. In this article I will contribute to his research by means of a discussion of one very famous example of case-based philosophy of science, namely the debate on the London & London model of superconductivity between Cartwright, Su´arezand Shomar on the one hand, and French, Ladyman, Bueno and Da Costa on the other. This debate has been going on for years, without any satisfactory resolution. I will argue here that this is because both sides impose on the historical case a particular philosophical conception of scientific representation that does not do justice to the historical facts. Both sides assume, more specifically, that the case concerns the discovery of a representational connection between a given experimental insight { the Meissner effect { and the diamagnetic meaning of London and London's new equations of superconductivity. I will show, however, that at the time of the Londons' publication, neither the experimental insight nor the meaning of the new equations was established: both were open The author would like to acknowledge the Research Foundation { Flanders (FWO) as funding institution. Center for Philosophical Psychology, Department of Philosophy, University of Antwerp, Grote Kauwenberg 18, 2000 Antwerpen, Belgium E-mail: [email protected] 2 Jan Potters for discussion and they were stabilized only later.
    [Show full text]
  • How Alfven's Theorem Explains the Meissner Effect
    May 15, 2020 13:36 MPLB S0217984920503005 page 1 2nd Reading Modern Physics Letters B 2050300 (31 pages) © World Scientific Publishing Company DOI: 10.1142/S0217984920503005 How Alfven's theorem explains the Meissner effect J. E. Hirsch Department of Physics, University of California, San Diego, La Jolla, CA 92093-0319, USA Received 4 March 2020 Revised 13 April 2020 Accepted 17 April 2020 Published 16 May 2020 Alfven's theorem states that in a perfectly conducting fluid magnetic field lines move with the fluid without dissipation. When a metal becomes superconducting in the presence of a magnetic field, magnetic field lines move from the interior to the surface (Meissner effect) in a reversible way. This indicates that a perfectly conducting fluid is flowing outward. I point this out and show that this fluid carries neither charge nor mass, but carries effective mass. This implies that the effective mass of carriers is lowered when a system goes from the normal to the superconducting state, which agrees with the prediction of the unconventional theory of hole superconductivity and with optical experiments in some superconducting materials. The 60-year old conventional understanding of the Meissner effect ignores Alfven's theorem and for that reason I argue that it does not provide a valid understanding of real superconductors. Keywords: Alfven's theorem; Meissner effect; frozen field lines. Mod. Phys. Lett. B Downloaded from www.worldscientific.com PACS Number(s): 74.20.-z; 52.30.Cv 1. Introduction When a conducting fluid moves, magnetic field lines tend to move with the fluid, as a consequence of Faraday's law.1 If the fluid is perfectly conducting, the lines are \frozen" in the fluid.
    [Show full text]
  • 1D Quantum Rings and Persistent Currents
    Introduction and motivation 1D quantum rings - theoretical approach Experimental survey Summary 1D quantum rings and persistent currents Anatoly Danilevich Lehrstuhl für Theoretische Festkörperphysik Institut für Theoretische Physik IV Universität Erlangen-Nürnberg March 9, 2007 Anatoly Danilevich 1D quantum rings and persistent currents Introduction and motivation 1D quantum rings - theoretical approach Experimental survey Summary Motivation In the last decades there was a growing interest for such microscopic systems like quantum rings because of few aspects: New physics behind quantum rigs, like Aharonov-Bohm-Effect and the possibility of controlling many-body systems with a well defined number of charge-carriers Possibility of the experimental realization of such systems, especially semiconductor technology developing new electronic devices Anatoly Danilevich 1D quantum rings and persistent currents Introduction and motivation 1D quantum rings - theoretical approach Experimental survey Summary Motivation In the last decades there was a growing interest for such microscopic systems like quantum rings because of few aspects: New physics behind quantum rigs, like Aharonov-Bohm-Effect and the possibility of controlling many-body systems with a well defined number of charge-carriers Possibility of the experimental realization of such systems, especially semiconductor technology developing new electronic devices Anatoly Danilevich 1D quantum rings and persistent currents Introduction and motivation 1D quantum rings - theoretical approach
    [Show full text]
  • On Magnetic Flux Trapping in Superconductors R
    TUOP08 Proceedings of LINAC2016, East Lansing, MI, USA ON MAGNETIC FLUX TRAPPING IN SUPERCONDUCTORS R. G. Eichhorn†, J. Hoke, Z. Mayle, CLASSE, Cornell University, Ithaca, USA Abstract Our research now indicates a third factor: the orientation Magnetic flux trapping on cool-down has become an im- of the magnetic field with respect to the superconductor portant factor in the performance of superconducting cavi- surface. ties. We have conducted systematic flux trapping experi- ments on samples to investigate the role of the orientation EXPERIMENTAL SET-UP of an ambient magnetic field relative to the niobium’s sur- To investigate the role of field orientation in flux trap- face. ping we designed a simple and flexible set-up. It consisted of a sample, clamped into position by a copper frame, two INTRODUCTION solenoids that could either be oriented parallel or orthogo- According to the perfect Meissner effect, a superconduc- nal to the sample, 4 flux gate sensors to measure magnetic tor is expected to expel all magnetic flux when it becomes field components (axial with respect to the sensor) and two superconducting. However, it is well known that supercon- cernox sensors to measure the temperature of the sample ducting cavities made from Niobium trap some of the mag- during the thermal cycle. Details of the set-up are shown in netic flux during the transitions to its superconducting state Fig. 1. while being exposed to an external magnetic field. The The solenoids consisted of wire coiled 25 times around trapped flux will result in normal conducting vortices, cylinders of stainless steel. Each solenoid had a diameter which add significantly to the total surface resistance.
    [Show full text]
  • Quantum Computing with Superconducting Devices
    6.763 Applied Superconductivity Lecture 1 Terry P. Orlando Dept. of Electrical Engineering MIT September 4, 2003 Outline • What is a Superconductor? • Discovery of Superconductivity • Meissner Effect • Type I Superconductors • Type II Superconductors • Theory of Superconductivity • Tunneling and the Josephson Effect • High-Temperature Supercondutors • Applications of Superconductors Massachusetts Institute of Technology What is a Superconductor? “A Superconductor has ZERO electrical resistance BELOW a certain critical temperature. Once set in motion, a persistent electric current will flow in the superconducting loop FOREVER without any power loss.” Magnetic Flux explusion A Superconductor EXCLUDES any magnetic fields that come near it. Massachusetts Institute of Technology How “Cool” are Superconductors? Below 77 Kelvin (-200 ºC): • Some Copper Oxide Ceramics superconduct Below 4 Kelvin (-270 ºC): • Some Pure Metals e.g. Lead, Mercury, Niobium superconduct Keeping at 77 K Keeping at 4K Keeping at 0 ºC Massachusetts Institute of Technology The Discovery of Superconductivity 1911 The Nobel Prize in Physics 1913 "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium" Heike Kamerlingh Onnes the Netherlands Leiden University Leiden, the Netherlands b. 1853 d. 1926 •http://www.nobel.se/physics/laureates Discovery of Superconductivity “As has been said, the experiment left no doubt that, as far as accuracy of measurement went, the resistance disappeared. At the same time, however, something unexpected occurred. The disappearance did not take place gradually but (compare Fig. 17) abruptly. From 1/500 the resistance at 4.2oK drop to a millionth part. At the lowest temperature, 1.5oK, it could be established that the resistance had Resistance become less than a thousand-millionth part of that at normal temperature.
    [Show full text]