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Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles
Measurements of quantum oscillations in the cuprate superconductors afford a
new opportunity to assess the extent to which the electronic properties of
these materials yield to a description rooted in Fermi liquid theory. However,
such an analysis is hampered by the small number of oscillatory periods
observed. Here we employ a genetic algorithm to globally model the field,
angular, and temperature dependence of the quantum oscillations observed in the
resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set
to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key
feature of the data is the first identification of the effect of Zeeman
splitting, which separates spin-up and spin-down contributions, indicating that
the quasiparticles in the cuprates behave as nearly free spins, constraining
the source of the Fermi surface reconstruction to something other than a
conventional spin density wave with moments parallel to the CuO2 planes.Comment: 8 pages, 4 figure
Subjective probability and quantum certainty
In the Bayesian approach to quantum mechanics, probabilities--and thus
quantum states--represent an agent's degrees of belief, rather than
corresponding to objective properties of physical systems. In this paper we
investigate the concept of certainty in quantum mechanics. Particularly, we
show how the probability-1 predictions derived from pure quantum states
highlight a fundamental difference between our Bayesian approach, on the one
hand, and Copenhagen and similar interpretations on the other. We first review
the main arguments for the general claim that probabilities always represent
degrees of belief. We then argue that a quantum state prepared by some physical
device always depends on an agent's prior beliefs, implying that the
probability-1 predictions derived from that state also depend on the agent's
prior beliefs. Quantum certainty is therefore always some agent's certainty.
Conversely, if facts about an experimental setup could imply agent-independent
certainty for a measurement outcome, as in many Copenhagen-like
interpretations, that outcome would effectively correspond to a preexisting
system property. The idea that measurement outcomes occurring with certainty
correspond to preexisting system properties is, however, in conflict with
locality. We emphasize this by giving a version of an argument of Stairs [A.
Stairs, Phil. Sci. 50, 578 (1983)], which applies the Kochen-Specker theorem to
an entangled bipartite system.Comment: 20 pages RevTeX, 1 figure, extensive changes in response to referees'
comment
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