71 research outputs found
Origin and tuning of the magnetocaloric effect for the magnetic refrigerant MnFe(P1-xGex)
Neutron diffraction and magnetization measurements of the magneto refrigerant
Mn1+yFe1-yP1-xGex reveal that the ferromagnetic and paramagnetic phases
correspond to two very distinct crystal structures, with the magnetic entropy
change as a function of magnetic field or temperature being directly controlled
by the phase fraction of this first-order transition. By tuning the physical
properties of this system we have achieved a maximum magnetic entropy change
exceeding 74 J/Kg K for both increasing and decreasing field, more than twice
the value of the previous record.Comment: 6 Figures. One tabl
Magnetic Structure and Properties of the S = 5/2 Triangular Antiferromagnet -NaFeO
The magnetic properties of -NaFeO are studied by neutron
diffraction and magnetization measurements. An ordered phase with spins aligned
along the b axis exists at low temperatures (T < 4 K). At intermediate
temperatures (4 K < T < 11 K), the system passes through an incommensurate
ordered phase before transforming into a short range ordered state at higher
temperatures that persists up to at least 50 K. Although the short range
ordering does not persist to room temperature according to neutron diffraction,
the magnetic susceptibility does not follow Curie-Weiss behavior, even up to
320 K. This rich magnetic behavior can be understood qualitatively as a
competition between different magnetic exchange interactions that are similar
in magnitude. The delicate balance between these interactions makes
-NaFeO a candidate for more detailed theoretical work to understand
magnetic behavior in frustrated magnetic systems.Comment: Submitted to Phys Rev B, 7 pages, 8 figure
Physical properties of FeSeTe single crystals grown under different conditions
We report on structural, magnetic, conductivity, and thermodynamic studies of
FeSeTe single crystals grown by self-flux and Bridgman methods.
The samples were prepared from starting materials of different purity at
various temperatures and cooling rates. The lowest values of the susceptibility
in the normal state, the highest transition temperature of 14.5 K, and
the largest heat-capacity anomaly at were obtained for pure (oxygen-free)
samples. The critical current density of A/cm (at 2
K) achieved in pure samples is attributed to intrinsic inhomogeneity due to
disorder at the cation and anion sites. The impure samples show increased
up to A/cm due to additional pinning centers of
FeO. The upper critical field of kOe is estimated
from the resistivity study in magnetic fields parallel to the \emph{c}-axis.
The anisotropy of the upper critical field reaches a value at . Extremely low values of the residual Sommerfeld coefficient for pure
samples indicate a high volume fraction of the superconducting phase (up to
97%). The electronic contribution to the specific heat in the superconducting
state is well described within a single-band BCS model with a temperature
dependent gap K. A broad cusp-like anomaly in the electronic
specific heat of samples with suppressed bulk superconductivity is ascribed to
a splitting of the ground state of the interstitial Fe ions. This
contribution is fully suppressed in the ordered state in samples with bulk
superconductivity.Comment: 11 pages, 11 figures, 3 table
Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity
We use neutron scattering to study the structural and magnetic phase
transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a
semimetal to a high-transition-temperature (high-Tc) superconductor through
Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural
lattice distortion followed by a stripe like commensurate antiferromagnetic
order with decreasing temperature. With increasing Fluorine doping, the
structural phase transition decreases gradually while the antiferromagnetic
order is suppressed before the appearance of superconductivity, resulting an
electronic phase diagram remarkably similar to that of the high-Tc copper
oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other
Fe-based superconductors reveals that the effective electronic band width
decreases systematically for materials with higher Tc. The results suggest that
electron correlation effects are important for the mechanism of high-Tc
superconductivity in these Fe pnictides.Comment: 19 pages, 5 figure
Formation of a simple cubic antiferromagnet through charge ordering in a double Dirac material
The appearance of spontaneous charge order in chemical systems is often
associated with the emergence of novel, and useful, properties. Here we show
through single crystal diffraction that the Eu ions in the mixed valent metal
EuPdS undergo long-range charge ordering at resulting in simple cubic lattices of Eu () and
Eu () ions. As only one of the two sublattices has a non-magnetic
ground state, the charge order results in the emergence of remarkably simple
G-type antiferromagnetic order at ,
observed in magnetization, specific heat, and neutron diffraction. Application
of a field is sufficient to induce a spin flop transition to
a magnetically polarized, but still charge ordered, state. Density functional
theory calculations show that this charge order also modifies the electronic
degeneracies present in the material: without charge order, EuPdS is an
example of a double Dirac material containing 8-fold degenerate electronic
states, greater than the maximum degeneracy of six possible in molecular
systems. The symmetry reduction from charge order transmutes 8-fold double
Dirac states into 4-fold Dirac states, a degeneracy that can be preserved even
in the presence of the magnetic order. Our results show not only how charge
order can be used to produce interesting magnetic lattices, but also how it can
be used to engineer controlled degeneracies in electronic states.Comment: 33 pages, 23 figure
Superconductivity at 36 K in beta-Fe1.01Se with the compression of the interlayer separation under pressure
In this letter, we report that the superconductivity transition temperature
in beta-Fe1.01Se increases from 8.5 to 36.7 K under applied pressure of 8.9
GPa. It then decreases at higher pressure. A dramatic change in volume is
observed at the same time Tc rises, due to a collapse of the separation between
the Fe2Se2 layers. A clear transition to a linear resistivity normal state is
seen on cooling at all pressures. No static magnetic ordering is observed for
the whole p-T phase diagram. We also report that at higher pressure (starting
around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal
NiAs-type structure and displays non-magnetic, insulating behavior. The
inclusion of electron correlation in band structure caculations is necessary to
describe this behavior, signifying that such correlations are important in this
chemical system. Our results strongly support unconventional superconductivity
in beta-Fe1.01Se.Comment: 17 pages, 4 figure
The Disabilities of the Arm, Shoulder and Hand Questionnaire (DASH) can measure the impairment, activity limitations and participation restriction constructs from the International Classification of Functioning, Disability and Health (ICF)
Background The International Classification of Functioning, Disability and Health (ICF) model of the consequences of disease identifies three health outcomes, impairment, activity limitations and participation restrictions. However, few orthopaedic health outcome measures were developed with reference to the ICF. This study examined the ability of a valid and frequently used measure of upper limb function, namely the Disabilities of the Arm, Shoulder and Hand Questionnaire (DASH), to operationalise the ICF. Methods Twenty-four judges used the method of Discriminant Content Validation to allocate the 38 items of the DASH to the theoretical definition of one or more ICF outcome. One-sample t-tests classified each item as measuring, impairment, activity limitations, participation restrictions, or a combination thereof. Results The DASH contains items able to measure each of the three ICF outcomes with discriminant validity. The DASH contains five pure impairment items, 19 pure activity limitations items and three participation restriction items. In addition, seven items measured both activity limitations and participation restrictions. Conclusions The DASH can measure the three health outcomes identified by the ICF. Consequently the DASH could be used to examine the impact of trauma and subsequent interventions on each health outcome in the absence of measurement confound
The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs
We report the temperature dependent x-ray powder diffraction of the
quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K
and 95 K. We have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with
Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150
K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural
phase transition is strongly reduced, by about ~30K, by increasing the internal
chemical pressure going from LaOFeAs to NdOFeAs. In contrast the
superconducting critical temperature increases from 27 K to 51 K going from
LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped
orthorhombic Cmma phase competes with the superconducting tetragonal phase.
Therefore by controlling the internal chemical pressure in new materials it
should be possible to push toward zero the critical temperature T0 of the
structural phase transition, giving the striped phase, in order to get
superconductors with higher Tc.Comment: 9 pages, 3 figure
An organizational framework and strategic implementation for system-level change to enhance research-based practice: QUERI Series
<p>Abstract</p> <p>Background</p> <p>The continuing gap between available evidence and current practice in health care reinforces the need for more effective solutions, in particular related to organizational context. Considerable advances have been made within the U.S. Veterans Health Administration (VA) in systematically implementing evidence into practice. These advances have been achieved through a system-level program focused on collaboration and partnerships among policy makers, clinicians, and researchers.</p> <p>The Quality Enhancement Research Initiative (QUERI) was created to generate research-driven initiatives that directly enhance health care quality within the VA and, simultaneously, contribute to the field of implementation science. This paradigm-shifting effort provided a natural laboratory for exploring organizational change processes. This article describes the underlying change framework and implementation strategy used to operationalize QUERI.</p> <p>Strategic approach to organizational change</p> <p>QUERI used an evidence-based organizational framework focused on three contextual elements: 1) cultural norms and values, in this case related to the role of health services researchers in evidence-based quality improvement; 2) capacity, in this case among researchers and key partners to engage in implementation research; 3) and supportive infrastructures to reinforce expectations for change and to sustain new behaviors as part of the norm. As part of a QUERI Series in <it>Implementation Science</it>, this article describes the framework's application in an innovative integration of health services research, policy, and clinical care delivery.</p> <p>Conclusion</p> <p>QUERI's experience and success provide a case study in organizational change. It demonstrates that progress requires a strategic, systems-based effort. QUERI's evidence-based initiative involved a deliberate cultural shift, requiring ongoing commitment in multiple forms and at multiple levels. VA's commitment to QUERI came in the form of visionary leadership, targeted allocation of resources, infrastructure refinements, innovative peer review and study methods, and direct involvement of key stakeholders. Stakeholders included both those providing and managing clinical care, as well as those producing relevant evidence within the health care system. The organizational framework and related implementation interventions used to achieve contextual change resulted in engaged investigators and enhanced uptake of research knowledge. QUERI's approach and progress provide working hypotheses for others pursuing similar system-wide efforts to routinely achieve evidence-based care.</p
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