3,319 research outputs found
Equilibrium entanglement vanishes at finite temperature
We show that the equilibrium entanglement of a bipartite system having a
finite number of quantum states vanishes at finite temperature, for arbitrary
interactions between its constituents and with the environment.Comment: 2 pages, no figures, first submitted on July 22, 200
Phase relationship between the long-time beats of free induction decays and spin echoes in solids
Recent theoretical work on the role of microscopic chaos in the dynamics and
relaxation of many-body quantum systems has made several experimentally
confirmed predictions about the systems of interacting nuclear spins in solids,
focusing, in particular, on the shapes of spin echo responses measured by
nuclear magnetic resonance (NMR). These predictions were based on the idea that
the transverse nuclear spin decays evolve in a manner governed at long times by
the slowest decaying eigenmode of the quantum system, analogous to a chaotic
resonance in a classical system. The present paper extends the above
investigations both theoretically and experimentally. On the theoretical side,
the notion of chaotic eigenmodes is used to make predictions about the
relationships between the long-time oscillation phase of the nuclear free
induction decay (FID) and the amplitudes and phases of spin echoes. On the
experimental side, the above predictions are tested for the nuclear spin decays
of F-19 in CaF2 crystals and Xe-129 in frozen xenon. Good agreement between the
theory and the experiment is found.Comment: 20 pages, 9 figures, significant new experimental content in
comparison with version
Asymptotic and intermediate long-time behavior of nuclear free induction decays in polycrystalline solids and powders
journal articleFree induction decay (FID) measured by nuclear magnetic resonance (NMR) in a polycrystalline solid is the isotropic average of the FIDs for individual single crystallites. It has been recently proposed theoretically and verified experimentally that the long-time behavior of single-crystal FIDs has the universal form of exponentially decaying sinusoidal oscillations. Polycrystalline averaging complicates the situation theoretically, while the available experimental evidence is also ambiguous. Exponentially decaying sinusoidal oscillations have been observed for 129Xe in polycrystalline solid xenon but not for 19F in the powder of CaF2. In this paper, we present the first principles FID calculations for the powders of both CaF2 and solid xenon. In both cases, the asymptotic long-time behavior has the expected form of exponentially decaying sinusoidal oscillations, which is determined by the single crystallite FID with the slowest exponential decay. However, this behavior appears only at rather small values of the signal that have not yet been measured in experiments. At intermediate times accessible experimentally, a polycrystalline FID depends on the distribution of the exponential decay constants and oscillation frequencies for single crystallite FIDs. In CaF2, these parameters are relatively broadly distributed, and as a result, the sinusoidal long-time oscillations become somewhat washed out. In contrast, the single crystallite parameters are more clustered in solid xenon, and, as a result, the experimentally observable range is characterized by well-defined oscillation frequency and exponential decay constant even though both of these parameters do not represent the true long-time behavior. The above difference of the intermediate FID behavior originates from the difference of the crystal structures of solid xenon and CaF2
Dimensionality of spin modulations in 1/8-doped lanthanum cuprates from the perspective of NQR and muSR experiments
We investigate the dimensionality of inhomogeneous spin modulation patterns
in the cuprate family of high-temperature superconductors with particular focus
on 1/8-doped lanthanum cuprates. We compare one-dimensional stripe modulation
pattern with two-dimensional checkerboard of spin vortices in the context of
nuclear quadrupole resonance(NQR) and muon spin rotation(muSR) experiments. In
addition, we also consider the third pattern, a two-dimensional superposition
of spin spirals. Overall, we have found that none of the above patterns leads
to a consistent interpretation of the two types of experiments considered.
This, in particular, implies that the spin vortex checkerboard cannot be ruled
out on the basis of available NQR/muSR experimental results.Comment: 6 pages, 2 figure
A quantum group version of quantum gauge theories in two dimensions
For the special case of the quantum group we present an alternative approach to quantum gauge theories in
two dimensions. We exhibit the similarities to Witten's combinatorial approach
which is based on ideas of Migdal. The main ingredient is the Turaev-Viro
combinatorial construction of topological invariants of closed, compact
3-manifolds and its extension to arbitrary compact 3-manifolds as given by the
authors in collaboration with W. Mueller.Comment: 6 pages (plain TeX
Chaotic properties of spin lattices near second-order phase transitions
We perform a numerical investigation of the Lyapunov spectra of chaotic
dynamics in lattices of classical spins in the vicinity of second-order
ferromagnetic and antiferromagnetic phase transitions. On the basis of this
investigation, we identify a characteristic of the shape of the Lyapunov
spectra, the "G-index", which exhibits a sharp peak as a function of
temperature at the phase transition, provided the order parameter is capable of
sufficiently strong dynamic fluctuations. As a part of this work, we also
propose a general numerical algorithm for determining the temperature in
many-particle systems, where kinetic energy is not defined.Comment: 9 pages, 11 figure
Post-Truth: An Alumni Economist’s Perspective
Drawing upon 50 years as an academic economist, this lecture to alumni of SOAS’s Department of Economics reflects upon the continual ‘post-truth’ aspects of mainstream economics, ranging over its substantive, if shifting, content, and its treatment of methodology and interdisciplinarity. It draws upon a wide range of theory, empirical analysis, policy and anecdote to highlight both the need for alternatives and the continuing, even increased, failure of the mainstream to engage with criticism and alternatives
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