851 research outputs found
Quark Masses: An Environmental Impact Statement
We investigate worlds that lie on a slice through the parameter space of the
Standard Model over which quark masses vary. We allow as many as three quarks
to participate in nuclei, while fixing the mass of the electron and the average
mass of the lightest baryon flavor multiplet. We classify as "congenial" worlds
that satisfy the environmental constraint that the quark masses allow for
stable nuclei with charges one, six, and eight, making organic chemistry
possible. Whether a congenial world actually produces observers depends on a
multitude of historical contingencies, beginning with primordial
nucleosynthesis, which we do not explore. Such constraints may be independently
superimposed on our results. Environmental constraints such as the ones we
study may be combined with information about the a priori distribution of quark
masses over the landscape of possible universes to determine whether the
measured values of the quark masses are determined environmentally, but our
analysis is independent of such an anthropic approach.
We estimate baryon masses as functions of quark masses and nuclear masses as
functions of baryon masses. We check for the stability of nuclei against
fission, strong particle emission, and weak nucleon emission. For two light
quarks with charges 2/3 and -1/3, we find a band of congeniality roughly 29 MeV
wide in their mass difference. We also find another, less robust region of
congeniality with one light, charge -1/3 quark, and two heavier, approximately
degenerate charge -1/3 and 2/3 quarks. No other assignment of light quark
charges yields congenial worlds with two baryons participating in nuclei. We
identify and discuss the region in quark-mass space where nuclei would be made
from three or more baryon species.Comment: 40 pages, 16 figures (in color), 4 tables. See paper for a more
detailed abstract. v4: Cleaning up minor typo
Z topology and superconductivity from symmetry lowering of a 3D Dirac Metal AuPb
3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons
and exhibit exotic physical properties It has been suggested that structurally
distorting a DSM can create a Topological Insulator (TI), but this has not yet
been experimentally verified. Furthermore, quasiparticle excitations known as
Majorana Fermions have been theoretically proposed to exist in materials that
exhibit superconductivity and topological surface states. Here we show that the
cubic Laves phase AuPb has a bulk Dirac cone above 100 K that gaps out upon
cooling at a structural phase transition to create a topologically non trivial
phase that superconducts below 1.2 K. The nontrivial Z = -1 invariant in
the low temperature phase indicates that AuPb in its superconducting state
must have topological surface states. These characteristics make AuPb a
unique platform for studying the transition between bulk Dirac electrons and
topological surface states as well as studying the interaction of
superconductivity with topological surface states
Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na2IrO3
Heisenberg interactions are ubiquitous in magnetic materials and have been
prevailing in modeling and designing quantum magnets. Bond-directional
interactions offer a novel alternative to Heisenberg exchange and provide the
building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as
its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential
realizations of the Kitaev model, and their reported magnetic behaviors may be
interpreted within the Kitaev framework. However, the extent of their relevance
to the Kitaev model remains unclear, as evidence for bond-directional
interactions remains indirect or conjectural. Here, we present direct evidence
for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and
show that they lead to strong magnetic frustration. Diffuse magnetic x-ray
scattering reveals broken spin-rotational symmetry even above Neel temperature,
with the three spin components exhibiting nano-scale correlations along
distinct crystallographic directions. This spin-space and real-space
entanglement directly manifests the bond-directional interactions, provides the
missing link to Kitaev physics in honeycomb iridates, and establishes a new
design strategy toward frustrated magnetism.Comment: Nature Physics, accepted (2015
Non-coplanar and counter-rotating incommensurate magnetic order stabilized by Kitaev interactions in -Li2IrO3
Materials that realize Kitaev spin models with bond-dependent anisotropic
interactions have long been searched for, as the resulting frustration effects
are predicted to stabilize novel forms of magnetic order or quantum spin
liquids. Here we explore the magnetism of -LiIrO, which has the
topology of a 3D Kitaev lattice of inter-connected Ir honeycombs. Using
resonant magnetic x-ray diffraction we find a complex, yet highly-symmetric
incommensurate magnetic structure with non-coplanar and counter-rotating Ir
moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally
accounts for all key features of the observed magnetic structure. Our results
provide strong evidence that -LiIrO realizes a spin Hamiltonian
with dominant Kitaev interactions.Comment: 10 pages, 7 figure
Transport evidence for Fermi-arc mediated chirality transfer in the Dirac semimetal CdAs
Dirac semi-metals show a linear electronic dispersion in three dimension
described by two copies of the Weyl equation, a theoretical description of
massless relativistic fermions. At the surface of a crystal, the breakdown of
fermion chirality is expected to produce topological surface states without any
counterparts in high-energy physics nor conventional condensed matter systems,
the so-called "Fermi Arcs". Here we present Shubnikov-de Haas oscillations
involving the Fermi Arc states in Focused Ion Beam prepared microstructures of
CdAs. Their unusual magnetic field periodicity and dependence on sample
thickness can be well explained by recent theoretical work predicting novel
quantum paths weaving the Fermi Arcs together with chiral bulk states, forming
"Weyl orbits". In contrast to conventional cyclotron orbits, these are governed
by the chiral bulk dynamics rather than the common momentum transfer due to the
Lorentz force. Our observations provide evidence for direct access to the
topological properties of charge in a transport experiment, a first step
towards their potential application.Comment: 25 pages, 11 figures, final published versio
Simultaneous colorectal and hepatic procedures for colorectal cancer result in increased morbidity but equivalent mortality compared with colorectal or hepatic procedures alone: outcomes from the National Surgical Quality Improvement Program
AbstractBackgroundSimultaneous colorectal and hepatic surgery for colorectal cancer (CRC) is increasing as surgery becomes safer and less invasive. There is controversy regarding the morbidity associated with simultaneous, compared with separate or staged, resections.MethodsData for 2005–2008 from the National Surgical Quality Improvement Program (NSQIP) were used to compare morbidity after 19 925 colorectal procedures for CRC (CR group), 2295 hepatic resections for metastatic CRC (HEP group), and 314 simultaneous colorectal and hepatic resections (SIM group).ResultsAn increasing number of simultaneous resections were performed per year. Fewer major colorectal and liver resections were performed in the SIM than in the CR and HEP groups. Patients in the SIM group had a longer operative time and postoperative length of stay compared with those in either the CR or HEP groups. Simultaneous procedures resulted in higher rates of postoperative morbidity and major morbidity than CR procedures, but not HEP procedures. This difference was driven by higher rates of wound and organ space infections, and a greater incidence of septic shock. Mortality rates did not differ among the groups.ConclusionsHospitals in the NSQIP are performing more simultaneous colonic and hepatic resections for CRC. These procedures are associated with increases in operative time, length of stay and rate of perioperative complications. Simultaneous procedures do not, however, increase perioperative mortality
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