149 research outputs found
Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6
We present the results of measurements of the thermal conductivity
of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and
300 K and with the heat flow directed either along or perpendicular to the
chain direction. The analysis of the anisotropy of the heat transport allowed
for the identification of a small but non-negligible magnon contribution
along the chains, superimposed on the dominant phonon contribution
. At temperatures above about 100 K the energy diffusion constant
D_E(T), calculated from the data, exhibits similar features as
the spin diffusion constant D_S(T), previously measured by NMR. In this regime,
the behaviour of both transport parameters is consistent with a diffusion
process that is caused by interactions inherent to one-dimensional S=1 spin
systems.Comment: 6 pages, 4 figure
Ga-NMR local susceptibility of the kagome-based magnet SrCr_9pGa_(12-9p)O_19. A high temperature study
We report a high- Ga-NMR study in the kagome-based antiferromagnetic
compound SrCrGaO (), and present a
refined mean-field analysis of the high T local NMR susceptibility of Cr
frustrated moments. We find that the intralayer kagome coupling is K,
and the interlayer coupling through non-kagome Cr moments is K. The ratio confirms the common belief that
the frustrated entity is a pyrochlore slab.Comment: 8 pages, 4 figures Conference paper: Highly Frustrated Magnetism
2000, Waterloo (Canada) Submitted to Canadian Journal of Physic
Tuning the spin Hamiltonian of NENP by external pressure: a neutron scattering study
We report an inelastic neutron scattering study of antiferromagnetic spin
dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external
hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations
in NENP are dominated by a long-lived triplet mode with a gap which is split by
orthorhombic crystalline anisotropy into a lower doublet centered at
1.2meV and a singlet at 2.5meV.
With pressure we observe appreciable shifts in these levels, which move to
1.45 meV and
2.2meV. The dispersion of these modes in the crystalline c-direction
perpendicular to the chain was measured here for the first time, and can be
accounted for by an interchain exchange J'_c approximately 3e-4*J which changes
only slightly with pressure. Since the average gap value 1.64
meV remains almost unchanged with P, we conclude that in NENP the application
of external pressure does not affect the intrachain coupling J appreciably, but
does produce a significant decrease of the single-ion anisotropy constant from
D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa.Comment: LaTeX file nenp_p.tex, 10 pages, 1 table, 5 figures. Submitted to
Phys. Rev.
Order by Disorder and Energetic Selection of the Ground State in the XY Pyrochlore Antiferromagnet Er2Ti2O7. An Inelastic Neutron Scattering Study
Examples of materials where an "order by disorder" mechanism is at play to
select a particular ground state are scarce. It has recently been proposed,
however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most
convincing case of this mechanism. Observation of a spin gap at zone centers
has recently been interpreted as a corroboration of this physics. In this
paper, we argue, however, that the anisotropy generated by the
interaction-induced admixing between the crystal-field ground and excited
levels provides for an alternative mechanism. It especially predicts the
opening of a spin gap of about 15 micro-eV, which is of the same order of
magnitude as the one observed experimentally. We report new high resolution
inelastic neutron scattering data which can be well understood within this
scenario.Comment: This version corrects a mistake on the numerical values of the
exchange couplings in relation to the published versio
What does it take to learn in next generation learning spaces?
This chapter identifies the attributes that learners need in order to learn effectively in new technology rich educational environments. There are a number of different ways of synthesising the findings from this emerging literature which relies heavily on qualitative research. This chapter reports on a literature review which adopted a deliberately interpretative qualitative meta-analysis, synthesising the findings from 15 key studies. As such, the chapter demonstrates a way of reviewing and compiling current research. The synthesis resulted in the identification of six attributes that learners need to do well in next generation learning spaces. These are: engaged, connected, confident, adaptable, intentional and self-aware. Although some of these attributes are applicable to all learning contexts, those of being connected, confident, adaptable and intentional seem to be particularly important in learning in next generation learning spaces. The challenge is to design learning activities that encourage and reward the development of these attributes. The hope is that through both its findings and its method, this chapter provokes debate on what it now means to be a successful learner in today’s technology rich world
Absence of gap for infinite half--integer spin ladders with an odd number of legs
A proof is presented for the absence of gap for spin ladders with an
odd number of legs, in the infinite leg length limit. This result is relevant
to the current discussion of coupled one--dimensional spin systems, a physical
realization of which are vanadyl pyrophosphate, (VO)PO, and
stoichiometric Sr Cu O (with ).Comment: REVTeX, 8 page
Customizable and scalable automated assessment of C/C++ programming assignments
The correction of exercises in programming courses is a laborious task that has traditionally been performed in a manual way. This situation, in turn, delays the access by students to feedback that can contribute significantly to their training as future professionals. Over the years, several approaches have been proposed to automate the assessment of students' programs. Static analysis is a known technique that can partially simulate the process of manual code review performed by lecturers. As such, it is a plausible option to assess whether students' solutions meet the requirements imposed on the assignments. However, implementing a personalized analysis beyond the rules included in existing tools may be a complex task for the lecturer without a mechanism that guides the work. In this paper, we present a method to provide automated and specific feedback to immediately inform students about their mistakes in programming courses. To that end, we developed the CAC++ library, which enables constructing tailored static analysis programs for C/C++ practices. The library allows for great flexibility and personalization of verifications to adjust them to each particular task, overcoming the limitations of most of the existing assessment tools. Our approach to providing specific feedback has been evaluated for a period of three academic years in a course related to object-oriented programming. The library allowed lecturers to reduce the size of the static analysis programs developed for this course. During this period, the academic results improved and undergraduates positively valued the aid offered when undertaking the implementation of assignments.Universidad de Cádiz, Grant/Award Numbers: sol-201500054192-tra, sol-201600064680-tra; Ministerio de Ciencia, Innovación y Universidades, Grant/Award Number: RTI2018-093608-B-C33; European Regional Development Fun
Strong-Coupling Theory of Rattling-Induced Superconductivity
In order to clarify the mechanism of the enhancement of superconducting
transition temperature due to anharmonic local oscillation of a
guest ion in a cage composed of host atoms, i.e., {\it rattling}, we analyze
the anharmonic Holstein model by applying the Migdal-Eliashberg theory. From
the evaluation of the normal-state electron-phonon coupling constant, it is
found that the strong coupling state is developed, when the bottom of a
potential for the guest ion becomes wide and flat. Then, is
enhanced with the increase of the anharmonicity in the potential, although
is rather decreased when the potential becomes a double-well type
due to very strong anharmonicity. From these results, we propose a scenario of
anharmonicity-controlled strong-coupling tendency for superconductivity induced
by rattling. We briefly discuss possible relevance of the present scenario with
superconductivity in -pyrochlore oxides.Comment: 8 pages, 6 figure
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