228 research outputs found
Commensurate to incommensurate magnetic phase transition in Honeycomb-lattice pyrovanadate Mn2V2O7
We have synthesized single crystalline sample of MnVO using
floating zone technique and investigated the ground state using magnetic
susceptibility, heat capacity and neutron diffraction. Our magnetic
susceptibility and heat capacity reveal two successive magnetic transitions at
19 K and 11.8 K indicating two distinct magnetically
ordered phases. The single crystal neutron diffraction study shows that in the
temperature () range 11.8 K 19 K the magnetic structure is
commensurate with propagation vector , while upon lowering
temperature below 11.8 K an incommensurate magnetic order emerges
with and the magnetic structure can be represented by
cycloidal modulation of the Mn spin in plane. We are reporting this
commensurate to incommensurate transition for the first time. We discuss the
role of the magnetic exchange interactions and spin-orbital coupling on the
stability of the observed magnetic phase transitions.Comment: 8 pages, 7 figure
Effect systems revisited—control-flow algebra and semantics
Effect systems were originally conceived as an inference-based program analysis to capture program behaviour—as a set of (representations of) effects. Two orthogonal developments have since happened. First, motivated by static analysis, effects were generalised to values in an algebra, to better model control flow (e.g. for may/must analyses and concurrency). Second, motivated by semantic questions, the syntactic notion of set- (or semilattice-) based effect system was linked to the semantic notion of monads and more recently to graded monads which give a more precise semantic account of effects.
We give a lightweight tutorial explanation of the concepts involved in these two threads and then unify them via the notion of an effect-directed semantics for a control-flow algebra of effects. For the case of effectful programming with sequencing, alternation and parallelism—illustrated with music—we identify a form of graded joinads as the appropriate structure for unifying effect analysis and semantics
Coeffects: A calculus of context-dependent computation
The notion of context in functional languages no longer refers just to variables in scope. Context can capture additional properties of variables (usage patterns in linear logics; caching requirements in dataflow languages) as well as additional resources or properties of the execution environment (rebindable resources; platform version in a cross-platform application). The recently introduced notion of coeffects captures the latter, whole-context properties, but it failed to capture fine-grained per-variable properties.
We remedy this by developing a generalized coeffect system with annotations indexed by a coeffect shape. By instantiating a concrete shape, our system captures previously studied flat (whole-context) coeffects, but also structural (per-variable) coeffects, making coeffect analyses more useful. We show that the structural system enjoys desirable syntactic properties and we give a categorical semantics using extended notions of indexed comonad.
The examples presented in this paper are based on analysis of established language features (liveness, linear logics, dataflow, dynamic scoping) and we argue that such context-aware properties will also be useful for future development of languages for increasingly heterogeneous and distributed platforms
Double superconducting transition in the filled skutterudite PrOs4Sb12 and sample characterizations
A thorough characterization of many samples of the filled skutterudite
compound PrOs4Sb12 is provided. We find that the double superconducting
transition in the specific heat Tc1~1.89K and Tc2~1.72K tends to appear in
samples with a large residual resistivity ratio, large specific heat jump at
the superconducting transition and with the highest absolute value of the
specific heat above Tc1. However, we present evidence which casts doubt on the
intrinsic nature of the double superconducting transition. The ratio of the two
specific heat jumps \Delta C(Tc1)/\Delta C(Tc2) shows a wide range of values on
crystals from different batches but also within the same batch. This ratio was
strongly reduced by polishing a sample down to 120um. Remarkably, three samples
exhibit a single sharp transition of ~15mK in width at Tc~1.7K. The normalized
specific heat jump (C-Cnormal)/Cnormal at Tc of two of them is higher than ~32%
so larger than the sum of the two specific heat jumps when a double transition
exists. As an evidence of better quality, the slope in the transition is at
least two time steeper.
We discuss the origins of the double transition; in particular we consider,
based on X-ray diffraction results, a scenario involving Pr-vacancies. The
superconducting phase diagram under magnetic field of a sample with a single
transition is fitted with a two-band model taking into account the good values
for the gap as deduced from thermal conductivity measurements.Comment: 10 pages, 9 figures, 2 tables, submitted to Physical review
Glass-like ordering and spatial inhomogeneity of magnetic structure in Ba3FeRu2O9 : The role of Fe/Ru-site disorder
Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9,
have been studied over a significant period of time in order to understand the
unusual magnetism of ruthenium metal. However, among them, the M=Fe compound
appears different since it is observed that unlike others, the 3d Fe ions and
4d Ru ions can easily exchange their crystallographic positions and as a result
many possible magnetic interactions become realizable. The present study
involving several experimental methods on this compound establish that the
magnetic structure of Ba3FeRu2O9 is indeed very different from all other 6H
ruthenates. Local structural study reveals that the possible Fe/Ru-site
disorder further extends to create local chemical inhomogeneity, affecting the
high temperature magnetism of this material. There is a gradual decrease of
57Fe M\"ossbauer spectral intensity with decreasing temperature (below 100 K),
which reveals that there is a large spread in the magnetic ordering
temperatures, corresponding to many spatially inhomogeneous regions. However,
finally at about 25 K, the whole compound is found to take up a global
glass-like magnetic ordering.Comment: 24 page, 7 figure
Adaptive modulation in Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy
The origin of incommensurate structural modulation in Ni-Mn based Heusler
type magnetic shape memory alloys (MSMAs) is still an unresolved issue inspite
of intense focus on this due to its role in the magnetic field induced
ultra-high strains. In the archetypal MSMA Ni2MnGa, the observation of
non-uniform displacement of atoms from their mean positions in the modulated
martensite phase, premartensite phase and charge density wave as well as the
presence of phason broadening of satellite peaks have been taken in support of
the electronic instability model linked with a soft acoustic phonon. We present
here results of a combined high resolution synchrotron x-ray powder diffraction
(SXRPD) and neutron powder diffraction (NPD) study on Ni2Mn1.4In0.6 using
(3+1)D superspace group approach, which reveal not only uniform atomic
displacements in the modulated structure of the martensite phase with
physically acceptable ordered magnetic moments in the antiferromagnetic phase
at low temperatures but also the absence of any premartensite phase and phason
broadening of the satellite peaks. Our HRTEM studies and first principles
calculations of the ground state also support uniform atomic displacements
predicted by powder diffraction studies. All these observations suggest that
the structural modulation in the martensite phase of Ni2Mn1.4In0.6 MSMA can be
explained in terms of the adaptive phase model. The present study underlines
the importance of superspace group analysis using complimentary SXRPD and NPD
in understanding the physics of the origin of modulation as well as the
magnetic and the modulated ground states of the Heusler type MSMAs. Our work
also highlights the fact that the mechanism responsible for the origin of
modulated structure in different Ni-Mn based MSMAs may not be universal and it
must be investigated thoroughly in different alloy compositions
Types from Data: Making Structured Data First-class Citizens in F#
Most modern applications interact with external services and access data in structured formats such as XML, JSON and CSV. Static type systems do not understand such formats, often making data access more cumbersome. Should we give up and leave the messy world of external data to dynamic typing and runtime checks? Of course, not! We present F# Data, a library that integrates external structured data into F#. As most real-world data does not come with an explicit schema, we develop a shape inference algorithm that infers a shape from representative sample documents. We then integrate the inferred shape into the F# type system using type providers. We formalize the process and prove a relative type soundness theorem. Our library significantly reduces the amount of data access code and it provides additional safety guarantees when contrasted with the widely used weakly typed techniques
Extended Call-by-Push-Value: Reasoning About Effectful Programs and Evaluation Order
Traditionally, reasoning about programs under varying evaluation regimes (call-by-value, call-by-name etc.) was done at the meta-level, treating them as term rewriting systems. Levy’s call-by-push-value (CBPV) calculus provides a more powerful approach for reasoning, by treating CBPV terms as a common intermediate language which captures both call-by-value and call-by-name, and by allowing equational reasoning about changes to evaluation order between or within programs.
We extend CBPV to additionally deal with call-by-need, which is non-trivial because of shared reductions. This allows the equational reasoning to also support call-by-need. As an example, we then prove that call-by-need and call-by-name are equivalent if nontermination is the only side-effect in the source language.
We then show how to incorporate an effect system. This enables us to exploit static knowledge of the potential effects of a given expression to augment equational reasoning; thus a program fragment might be invariant under change of evaluation regime only because of knowledge of its effects
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