1,512 research outputs found
Maxwell electromagnetism as an emergent phenomenon in condensed matter
The formulation of a complete theory of classical electromagnetism by Maxwell
is one of the milestones of science. The capacity of many-body systems to
provide emergent mini-universes with vacua quite distinct from the one we
inhabit was only recognised much later. Here, we provide an account of how
simple systems of localised spins manage to emulate Maxwell electromagnetism in
their low-energy behaviour. They are much less constrained by symmetry
considerations than the relativistically invariant electromagnetic vacuum, as
their substrate provides a non-relativistic background with even translational
invariance broken. They can exhibit rich behaviour not encountered in
conventional electromagnetism. This includes the existence of magnetic monopole
excitations arising from fractionalisation of magnetic dipoles; as well as the
capacity of disorder, by generating defects on the lattice scale, to produce
novel physics, as exemplified by topological spin glassiness or random Coulomb
magnetism.Comment: Talk at Royal Society Symposium, "Unifying Physics and Technology in
the Light of Maxwell's Equations", November 201
Proximity effect thermometer for local temperature measurements on mesoscopic samples
Using the strong temperature dependent resistance of a normal metal wire in
proximity to a superconductor, we have been able to measure the local
temperature of electrons heated by flowing a dc current in a metallic wire to
within a few tens of millikelvin at low temperatures. By placing two such
thermometers at different parts of a sample, we have been able to measure the
temperature difference induced by a dc current flowing in the sample. This
technique may provide a flexible means of making quantitative thermal and
thermoelectric measurements on mesoscopic metallic samples
Some problems of the calculation of three-dimensional boundary layer flows on general configurations
An accurate solution of the three-dimensional boundary layer equations over general configurations such as those encountered in aircraft and space shuttle design requires a very efficient, fast, and accurate numerical method with suitable turbulence models for the Reynolds stresses. The efficiency, speed, and accuracy of a three-dimensional numerical method together with the turbulence models for the Reynolds stresses are examined. The numerical method is the implicit two-point finite difference approach (Box Method) developed by Keller and applied to the boundary layer equations by Keller and Cebeci. In addition, a study of some of the problems that may arise in the solution of these equations for three-dimensional boundary layer flows over general configurations
Developing concepts for early mental health prevention and treatment using the built environment
Random Coulomb antiferromagnets: from diluted spin liquids to Euclidean random matrices
We study a disordered classical Heisenberg magnet with uniformly
antiferromagnetic interactions which are frustrated on account of their
long-range Coulomb form, {\em i.e.} in and in . This arises naturally as the limit of the
emergent interactions between vacancy-induced degrees of freedom in a class of
diluted Coulomb spin liquids (including the classical Heisenberg
antiferromagnets on checkerboard, SCGO and pyrochlore lattices) and presents a
novel variant of a disordered long-range spin Hamiltonian. Using detailed
analytical and numerical studies we establish that this model exhibits a very
broad paramagnetic regime that extends to very large values of in both
and . In , using the lattice-Green function based finite-size
regularization of the Coulomb potential (which corresponds naturally to the
underlying low-temperature limit of the emergent interactions between
orphan-spins), we only find evidence that freezing into a glassy state occurs
in the limit of strong coupling, , while no such transition seems to
exist at all in . We also demonstrate the presence and importance of
screening for such a magnet. We analyse the spectrum of the Euclidean random
matrices describing a Gaussian version of this problem, and identify a
corresponding quantum mechanical scattering problem.Comment: two-column PRB format; 17 pages; 24 .eps figure
Analytic Framework for Students' Use of Mathematics in Upper-Division Physics
Many students in upper-division physics courses struggle with the
mathematically sophisticated tools and techniques that are required for
advanced physics content. We have developed an analytical framework to assist
instructors and researchers in characterizing students' difficulties with
specific mathematical tools when solving the long and complex problems that are
characteristic of upper-division. In this paper, we present this framework,
including its motivation and development. We also describe an application of
the framework to investigations of student difficulties with direct integration
in electricity and magnetism (i.e., Coulomb's Law) and approximation methods in
classical mechanics (i.e., Taylor series). These investigations provide
examples of the types of difficulties encountered by advanced physics students,
as well as the utility of the framework for both researchers and instructors.Comment: 17 pages, 4 figures, 3 tables, in Phys. Rev. - PE
How periodic driving heats a disordered quantum spin chain
We study the energy absorption in real time of a disordered quantum spin chain subjected to coherent monochromatic periodic driving. We determine characteristic fingerprints of the well-known ergodic (Floquet-Eigenstate thermalization hypothesis for slow driving/weak disorder) and many-body localized (Floquet-many-body localization for fast driving/strong disorder) phases. In addition, we identify an intermediate regime, where the energy density of the system-unlike the entanglement entropy a local and bounded observable-grows logarithmically slowly over a very large time window
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