Ftf2 Flashcards
The elements Fe, Ni and Co order ferromagnetically with saturation moments that do not obey Hund’s rules for localized atomic moments. What is the reason for this and how do you instead describe the ferromagnetism in 3d metals?
Fe, Co and Ni are 3d metals. The 3d electrons are not well described in a localized model, but a bandmodel is used including separations of spin up and spin down. Below Tc there ios a shift of energy of the d-electrons with spin-up with respect to the energy of those with spin down implying a net magnetic moment. The magnetic moment/atom is not given by Hund’s rules (also not including quenching) but by the amount of energy and the shape of the d-bands.
Ionic crystals are opaque to electromagnetic radiation in a wave length interval in the infra red. What is the mechanism behind this and what kind of phonons are excited in the crystal in the process?
When the energy of the incoming photons is of the same magnitude as the energy if optical phonons, they excite transverse optical phonons and the most of the ratiation is absorbed or reflected by the ionic crystal.
How do you (in the simplest way) measure the static dielectric constant of an insulating material?
Measure the capacitance of a plate capacitor with (Cm) and without (C0) the material plates. epsilon = Cm/C0
The BCS theory explains superconductivity. The current transport in an ordinary metal occurs via the motion of single electrons where as electron pairs (Cooper pairs) are responsible for charge transport in superconductors. From a quantum mechanical - statistical physics point of view, what is the fundamental difference between the description of a single electron and a Cooper pair? What interaction mechanism is behind the formation of Cooper pairs and how is this mechanism manifested in the critical temperature of a superconducting element?
Single electrons are fermions (fermions have odd half-integer spin (1⁄2, 3⁄2, 5⁄2, …)), whereas electron pairs behave as bosons (bosons have integer value spin quantum numbers (0, 1, 2, …)). The pairs are created because of the electron-phonon interaction; revealed from isotope effect (when one of the atoms in the material is replaced by one of its isotopes).
The magnetic susceptibility of a thin film (thickness d) is lower thatn the ideal |x|=1. Draw in a magnetisation vs. field diagram magnetisation curves for a thin and a thick film (bulk like) up to their respective critical fields. Explain in words and/or a simple figure why the ratio between the penetration depth and the film thickness is the governing parameter for the magnitude of the susceptibility.
The magnetic field penetrates a superconductor with an exponential decay: Bf=Ba*e^(-x/lambda), where x is the distance from the surface of the superconductor. In a thin film, where the thickness of the film, d, is of the order of the penetration depth, lambda, the susceptibility of the film is diminished with respect to the ideal -1. A larger and larger volume of the film is penetrated by the field the thinner the film and thus the ratio lambda/d determines the magnitude of the susceptibility.
Describe the temperature dependence of the magnetic susceptibility (a) a diamagnetic material, (b) a Curie paramagnetic material, (c) a material with metallic paramagnetism. It is advantageous to answer with a plot showing susceptibility vs. temperature.
(a) temperature independent and negative value of x
(b) temperature dependent and positive: x(T) = C/T
(c) temperature independent and positive value of x
The hysteresis loop describes the field dependence of the magnetisation of a ferromagnet. Sketch hysteresis curves for a hard magnetic (permanent magnet) and a soft magnetic (e.g. transformer sheet) material - which property (parameter) characterising the hysteresis curve differs some orders of magnitude between the two material types.
The hysteresis loops look quite similar for soft and hard magnets: however, the coercive force is very different, order 1 Oe or less for a soft magnet and order 1 kOe and larger for a hard magnet.
Which are the three polarisation mechanisms that can contribute to the static dielectric constant of an insulatior?
Electronic, ionic and dipolar contributions. The dipolar polarisation can follow an applied ac electric field up to microwave frequencies, the ionic contribution remains into the infra red frequency range and the electronic contribution disappears at hard X-ray frequencies.
Discuss the fact that good electrical conductors as Au, Ag and Cu remain normal conductors at low temperatures, whereas less good conductors such as Sn, In and Pb become superconducting at low enough temperature.
A worse electrical conductivity at room temperature indicates strong coupling (scattering probability) between electrons and phonons - the mechanism that causes superconductivity (formation of Cooper pairs) is given by coupling between electrons and lattice vibrations (phonons). In good electrical conductors this essential coupling mechanism is too weak.
Describe the difference between type 1 and type 2 superconductors - illustrate the difference by a magnetisation vs. field diagram
The magnetic field is expelled from the interior of type 1 superconductor, the susceptibility is equal to -1; at the critical field (Hc) the field penetrates the superconductor and the whole material becomes normal conducting. In type 2 the field is expelled up to the first critical field (Hc1), at higher fields the field penetrates the material in form of vortices (fluxlines, each carrying 1 fluxquantum fi0), at Hc2 the whole material becomes normal conducting.
A thin film (50nm) of iron has been grown on an MgO substrate. How will the magnetic moment of the domains be directed in a demagnetized sample at zero applied magnetic fields? In the film plane or out of the film plane? Motivate!
The magnetization will be confined to the film plane due to the geometric demagnetization effect: N=0 in-plane and N=1 out of plane.
Which are the mechanisms of polarization that may contribute to the static dielectric constant of an insulating material?
They are Electronic polarization, dipolar or Orientation polarization, Ionic polarization and Interfacial polarization.
What causes the spontaneous electric dipole moment of a ferroelectric crystal?
At temperatures below the ferroelectric transition temperature there occur collective non-symmetric displacements of ions with respect to an originally symmetric structure. This gives rise to a spontaneous electric dipole moment in the crystal.
A manufacturer of superconducting magnets (solenoids) is to choose new wires for her magnets. She has two different wires of superconducting material type 2 to choose between; one exhibiting weak pinning of fluxoids (vortices) and the other with strong pinning. Which of the wires should she choose and why?
The wire with much pinning should be chosen. In the other wire the vortices are free to move and this causes dissipation (meaning scattering ) when a current runs in the wire.
A small permanent magned Nd2Fe14B placed on top of a thick disc made of the high temperature superconductor material YBa2Vu3O7 (Tc =92K) levitates when the disc is cooled to 77K (liquid nitrogen temperature). What physical phenomenon explains the effect? And how does it work?
The Meissner effect prevents the magnetic field from the magnet to enter the superconductor. This can be pictured as if a magnet with reversed poles compared to the permanent magnet is created by the supercurrents causing the magnet to lift. Since the superconductor is of extreme 2, some flux will enter the material in form of vortices and these are pinned (wakly or strongly depending on the material) and hinder the magnet from falling off the superconductor.
[The Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. This expulsion will repel a nearby magnet.]
