203 research outputs found
Locally critical quantum phase transitions in strongly correlated metals
When a metal undergoes a continuous quantum phase transition, non-Fermi
liquid behaviour arises near the critical point. It is standard to assume that
all low-energy degrees of freedom induced by quantum criticality are spatially
extended, corresponding to long-wavelength fluctuations of the order parameter.
However, this picture has been contradicted by recent experiments on a
prototype system: heavy fermion metals at a zero-temperature magnetic
transition. In particular, neutron scattering from CeCuAu has
revealed anomalous dynamics at atomic length scales, leading to much debate as
to the fate of the local moments in the quantum-critical regime. Here we report
our theoretical finding of a locally critical quantum phase transition in a
model of heavy fermions. The dynamics at the critical point are in agreement
with experiment. We also argue that local criticality is a phenomenon of
general relevance to strongly correlated metals, including doped Mott
insulators.Comment: 20 pages, 3 figures; extended version, to appear in Natur
Thermodynamic analysis of the Quantum Critical behavior of Ce-lattice compounds
A systematic analysis of low temperature magnetic phase diagrams of Ce
compounds is performed in order to recognize the thermodynamic conditions to be
fulfilled by those systems to reach a quantum critical regime and,
alternatively, to identify other kinds of low temperature behaviors. Based on
specific heat () and entropy () results, three different types of
phase diagrams are recognized: i) with the entropy involved into the ordered
phase () decreasing proportionally to the ordering temperature
(), ii) those showing a transference of degrees of freedom from the
ordered phase to a non-magnetic component, with their jump
() vanishing at finite temperature, and iii) those ending in a
critical point at finite temperature because their do not decrease
with producing an entropy accumulation at low temperature.
Only those systems belonging to the first case, i.e. with as
, can be regarded as candidates for quantum critical behavior.
Their magnetic phase boundaries deviate from the classical negative curvature
below \,K, denouncing frequent misleading extrapolations down to
T=0. Different characteristic concentrations are recognized and analyzed for
Ce-ligand alloyed systems. Particularly, a pre-critical region is identified,
where the nature of the magnetic transition undergoes significant
modifications, with its discontinuity strongly
affected by magnetic field and showing an increasing remnant entropy at . Physical constraints arising from the third law at are discussed
and recognized from experimental results
Fermi surface instability at the hidden-order transition of URu2Si2
Solids with strong electron correlations generally develop exotic phases of
electron matter at low temperatures. Among such systems, the heavy-fermion
semi-metal URu2Si2 presents an enigmatic transition at To = 17.5 K to a `hidden
order' state whose order parameter remains unknown after 23 years of intense
research. Various experiments point to the reconstruction and partial gapping
of the Fermi surface when the hidden-order establishes. However, up to now, the
question of how this transition affects the electronic spectrum at the Fermi
surface has not been directly addressed by a spectroscopic probe. Here we show,
using angle-resolved photoemission spectroscopy, that a band of heavy
quasi-particles drops below the Fermi level upon the transition to the
hidden-order state. Our data provide the first direct evidence of a large
reorganization of the electronic structure across the Fermi surface of URu2Si2
occurring during this transition, and unveil a new kind of Fermi-surface
instability in correlated electron systemsComment: 15 pages, 5 figure
To use or not to use cool superconductors?
The high critical temperature and magnetic field in cuprates and Fe-based
superconductors are not enough to assure applications at higher temperatures.
Making these superconductors useful involves complex and expensive technologies
to address many conflicting physics and materials requirements
Emergent Rank-5 'Nematic' Order in URu2Si2
Novel electronic states resulting from entangled spin and orbital degrees of
freedom are hallmarks of strongly correlated f-electron systems. A spectacular
example is the so-called 'hidden-order' phase transition in the heavy-electron
metal URu2Si2, which is characterized by the huge amount of entropy lost at
T_{HO}=17.5K. However, no evidence of magnetic/structural phase transition has
been found below T_{HO} so far. The origin of the hidden-order phase transition
has been a long-standing mystery in condensed matter physics. Here, based on a
first-principles theoretical approach, we examine the complete set of multipole
correlations allowed in this material. The results uncover that the
hidden-order parameter is a rank-5 multipole (dotriacontapole) order with
'nematic' E^- symmetry, which exhibits staggered pseudospin moments along the
[110] direction. This naturally provides comprehensive explanations of all key
features in the hidden-order phase including anisotropic magnetic excitations,
nearly degenerate antiferromagnetic-ordered state, and spontaneous
rotational-symmetry breaking.Comment: See the published version with more detailed discussion
Imaging the Fano Lattice to 'Hidden Order' Transition in URu2Si2
Within a Kondo lattice, the strong hybridization between electrons localized
in real space (r-space) and those delocalized in momentum-space (k-space)
generates exotic electronic states called 'heavy fermions'. In URu2Si2 these
effects begin at temperatures around 55K but they are suddenly altered by an
unidentified electronic phase transition at To = 17.5 K. Whether this is
conventional ordering of the k-space states, or a change in the hybridization
of the r-space states at each U atom, is unknown. Here we use spectroscopic
imaging scanning tunnelling microscopy (SI-STM) to image the evolution of
URuSi2 electronic structure simultaneously in r-space and k-space. Above To,
the 'Fano lattice' electronic structure predicted for Kondo screening of a
magnetic lattice is revealed. Below To, a partial energy gap without any
associated density-wave signatures emerges from this Fano lattice.
Heavy-quasiparticle interference imaging within this gap reveals its cause as
the rapid splitting below To of a light k-space band into two new heavy fermion
bands. Thus, the URu2Si2 'hidden order' state emerges directly from the Fano
lattice electronic structure and exhibits characteristics, not of a
conventional density wave, but of sudden alterations in both the hybridization
at each U atom and the associated heavy fermion states.Comment: Main Article + Supplementary Informatio
Stores healthy options project in remote indigenous communities (SHOP@RIC): a protocol of a randomised trial promoting healthy food and beverage purchases through price discounts and in-store nutrition education
BackgroundIndigenous Australians suffer a disproportionate burden of preventable chronic disease compared to their non-Indigenous counterparts – much of it diet-related. Increasing fruit and vegetable intakes and reducing sugar-sweetened soft-drink consumption can reduce the risk of preventable chronic disease. There is evidence from some general population studies that subsidising healthier foods can modify dietary behaviour. There is little such evidence relating specifically to socio-economically disadvantaged populations, even though dietary behaviour in such populations is arguably more likely to be susceptible to such interventions.This study aims to assess the impact and cost-effectiveness of a price discount intervention with or without an in-store nutrition education intervention on purchases of fruit, vegetables, water and diet soft-drinks among remote Indigenous communities.Methods/DesignWe will utilise a randomised multiple baseline (stepped wedge) design involving 20 communities in remote Indigenous Australia. The study will be conducted in partnership with two store associations and twenty Indigenous store boards. Communities will be randomised to either i) a 20% price discount on fruit, vegetables, water and diet soft-drinks; or ii) a combined price discount and in-store nutrition education strategy. These interventions will be initiated, at one of five possible time-points, spaced two-months apart. Weekly point-of-sale data will be collected from each community store before, during, and for six months after the six-month intervention period to measure impact on purchasing of discounted food and drinks. Data on physical, social and economic factors influencing weekly store sales will be collected in order to identify important covariates. Intervention fidelity and mediators of behaviour change will also be assessed.DiscussionThis study will provide original evidence on the effectiveness and cost-effectiveness of price discounts with or without an in-store nutrition education intervention on food and drink purchasing among a socio-economically disadvantaged population in a real-life setting
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