69 research outputs found
Antiferromagnetic phase transition and spin correlations in NiO
We have investigated the antiferromagnetic (AF) phase transition and spin correlations in NiO by high-temperature neutron diffraction below and above T-N. We show that AF phase transition is a continuous second-order transition within our experimental resolution. The spin correlations manifested by the strong diffuse magnetic scattering persist well above T-N approximate to 530 K and could still be observed at T=800 K which is about 1.5T(N). We argue that the strong spin correlations above T-N are due to the topological frustration of the spins on a fcc lattice. The Neel temperature is substantially reduced by this process. We determined the critical exponents beta=0.328 +/- 0.002 and nu=0.64 +/- 0.03 and the Neel temperature T-N=530 +/- 1 K. These critical exponents suggest that NiO should be regarded as a 3dXY system
Magnon dispersion and thermodynamics in CsNiF_3
We present an accurate transfer matrix renormalization group calculation of
the thermodynamics in a quantum spin-1 planar ferromagnetic chain. We also
calculate the field dependence of the magnon gap and confirm the accuracy of
the magnon dispersion derived earlier through an 1/n expansion. We are thus
able to examine the validity of a number of previous calculations and further
analyze a wide range of experiments on CsNiF_3 concerning the magnon
dispersion, magnetization, susceptibility, and specific heat. Although it is
not possible to account for all data with a single set of parameters, the
overall qualitative agreement is good and the remaining discrepancies may
reflect departure from ideal quasi-one-dimensional model behavior. Finally, we
present some indirect evidence to the effect that the popular interpretation of
the excess specific heat in terms of sine-Gordon solitons may not be
appropriate.Comment: 9 pages 10 figure
Complete population transfer in a degenerate 3-level atom
We find conditions required to achieve complete population transfer, via
coherent population trapping, from an initial state to a designated final state
at a designated time in a degenerate 3-level atom, where transitions are caused
by an external interaction. Complete population transfer from an initially
occupied state 1 to a designated state 2 occurs under two conditions. First,
there is a constraint on the ratios of the transition matrix elements of the
external interaction. Second, there is a constraint on the action integral over
the interaction, or "area", corresponding to the phase shift induced by the
external interaction. Both conditions may be expressed in terms of simple odd
integers.Comment: 22 pages, 4 figure
Relativistic calculations of the lifetimes and hyperfine structure constants in Zn
This work presents accurate {\it ab initio} determination of the magnetic
dipole (M1) and electric quadrupole (E2) hyperfine structure constants for the
ground and a few low-lying excited states in Zn, which is one of
the interesting systems in fundamental physics. The coupled-cluster (CC) theory
within the relativistic framework has been used here in this calculations. Long
standing demands for a relativistic and highly correlated calculations like CC
can be able to resolve the disagreements among the lifetime estimations
reported previously for a few low-lying states of Zn. The role of
different electron correlation effects in the determination of these quantities
are discussed and their contributions are presented.Comment: 9 pages, 1 figure. submitted to J. Phys. B Fast Trac
Molecular dynamic simulation of a homogeneous bcc -> hcp transition
We have performed molecular dynamic simulations of a Martensitic bcc->hcp
transformation in a homogeneous system. The system evolves into three
Martensitic variants, sharing a common nearest neighbor vector along a bcc
direction, plus an fcc region. Nucleation occurs locally, followed by
subsequent growth. We monitor the time-dependent scattering S(q,t) during the
transformation, and find anomalous, Brillouin zone-dependent scattering similar
to that observed experimentally in a number of systems above the transformation
temperature. This scattering is shown to be related to the elastic strain
associated with the transformation, and is not directly related to the phonon
response.Comment: 11 pages plus 8 figures (GIF format); to appear in Phys. Rev.
Non-affirmative Theory of Education as a Foundation for Curriculum Studies, Didaktik and Educational Leadership
This chapter presents non-affirmative theory of education as the foundation for a new research program in education, allowing us to bridge educational leadership, curriculum studies and Didaktik. We demonstrate the strengths of this framework by analyzing literature from educational leadership and curriculum theory/didaktik. In contrast to both socialization-oriented explanations locating curriculum and leadership within existing society, and transformation-oriented models viewing education as revolutionary or super-ordinate to society, non-affirmative theory explains the relation between education and politics, economy and culture, respectively, as non-hierarchical. Here critical deliberation and discursive practices mediate between politics, culture, economy and education, driven by individual agency in historically developed cultural and societal institutions. While transformative and socialization models typically result in instrumental notions of leadership and teaching, non-affirmative education theory, previously developed within German and Nordic education, instead views leadership and teaching as relational and hermeneutic, drawing on ontological core concepts of modern education: recognition; summoning to self-activity and Bildsamkeit. Understanding educational leadership, school development and teaching then requires a comparative multi-level approach informed by discursive institutionalism and organization theory, in addition to theorizing leadership and teaching as cultural-historical and critical-hermeneutic activity. Globalisation and contemporary challenges to deliberative democracy also call for rethinking modern nation-state based theorizing of education in a cosmopolitan light. Non-affirmative education theory allows us to understand and promote recognition based democratic citizenship (political, economical and cultural) that respects cultural, ethical and epistemological variations in a globopolitan era. We hope an American-European-Asian comparative dialogue is enhanced by theorizing education with a non-affirmative approach
Mean field and Monte Carlo simulation studies of premartensitic effects in Ni2MnGa
We have extended the degenerate BEG model[l] to include magnetic degrees of freedom in order to study the premartensitic effects in Ni2MnGa. The model is solved by using mean-field theory and Monte Carlo techniques [2]. The numerical simulations reveal the crucial importance of fluctuations in pretransitional effects. Moreover, we find that a large variety of premartensitic effects may appear due to the magnetoelastic coupling. For large values of the coupling parameter a first-order transition line ending in a critical point appears. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling strength. In particular, the premartensitic transition occurs when such coupling is strong enough to prevent a complete softening of the involved phonon mode. The implication of the results in relation to the available experimental data is discussed
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