565 research outputs found
STM imaging of impurity resonances on BiSe
In this paper we present detailed study of the density of states near defects
in BiSe. In particular, we present data on the commonly found
triangular defects in this system. While we do not find any measurable
quasiparticle scattering interference effects, we do find localized resonances,
which can be well fitted by theory once the potential is taken to be extended
to properly account for the observed defects. The data together with the fits
confirm that while the local density of states around the Dirac point of the
electronic spectrum at the surface is significantly disrupted near the impurity
by the creation of low-energy resonance state, the Dirac point is not locally
destroyed. We discuss our results in terms of the expected protected surface
state of topological insulators.Comment: 5 pages, 6 figure
Social Influence and the Collective Dynamics of Opinion Formation
Social influence is the process by which individuals adapt their opinion,
revise their beliefs, or change their behavior as a result of social
interactions with other people. In our strongly interconnected society, social
influence plays a prominent role in many self-organized phenomena such as
herding in cultural markets, the spread of ideas and innovations, and the
amplification of fears during epidemics. Yet, the mechanisms of opinion
formation remain poorly understood, and existing physics-based models lack
systematic empirical validation. Here, we report two controlled experiments
showing how participants answering factual questions revise their initial
judgments after being exposed to the opinion and confidence level of others.
Based on the observation of 59 experimental subjects exposed to peer-opinion
for 15 different items, we draw an influence map that describes the strength of
peer influence during interactions. A simple process model derived from our
observations demonstrates how opinions in a group of interacting people can
converge or split over repeated interactions. In particular, we identify two
major attractors of opinion: (i) the expert effect, induced by the presence of
a highly confident individual in the group, and (ii) the majority effect,
caused by the presence of a critical mass of laypeople sharing similar
opinions. Additional simulations reveal the existence of a tipping point at
which one attractor will dominate over the other, driving collective opinion in
a given direction. These findings have implications for understanding the
mechanisms of public opinion formation and managing conflicting situations in
which self-confident and better informed minorities challenge the views of a
large uninformed majority.Comment: Published Nov 05, 2013. Open access at:
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.007843
Determination of the phase diagram of the electron doped superconductor Ba(FeCo)As
Systematic measurements of the resistivity, heat capacity, susceptibility and
Hall coefficient are presented for single crystal samples of the electron-doped
superconductor Ba(FeCo)As. These data delineate an
phase diagram in which the single magnetic/structural phase transition that is
observed for undoped BaFeAs at 134 K apparently splits into two
distinct phase transitions, both of which are rapidly suppressed with
increasing Co concentration. Superconductivity emerges for Co concentrations
above , and appears to coexist with the broken symmetry state for
an appreciable range of doping, up to . The optimal
superconducting transition temperature appears to coincide with the Co
concentration at which the magnetic/structural phase transitions are totally
suppressed, at least within the resolution provided by the finite step size
between crystals prepared with different doping levels. Superconductivity is
observed for a further range of Co concentrations, before being completely
suppressed for and above. The form of this phase diagram
is suggestive of an association between superconductivity and a quantum
critical point arising from suppression of the magnetic and/or structural phase
transitions
Ranking with social cues: Integrating online review scores and popularity information
Online marketplaces, search engines, and databases employ aggregated social
information to rank their content for users. Two ranking heuristics commonly
implemented to order the available options are the average review score and
item popularity-that is, the number of users who have experienced an item.
These rules, although easy to implement, only partly reflect actual user
preferences, as people may assign values to both average scores and popularity
and trade off between the two. How do people integrate these two pieces of
social information when making choices? We present two experiments in which we
asked participants to choose 200 times among options drawn directly from two
widely used online venues: Amazon and IMDb. The only information presented to
participants was the average score and the number of reviews, which served as a
proxy for popularity. We found that most people are willing to settle for items
with somewhat lower average scores if they are more popular. Yet, our study
uncovered substantial diversity of preferences among participants, which
indicates a sizable potential for personalizing ranking schemes that rely on
social information.Comment: 4 pages, 3 figures, ICWS
STM imaging of a bound state along a step on the surface of the topological insulator BiTe
Detailed study of the LDOS associated with the surface-state-band near a
step-edge of the strong topological-insulator Bi2Te3, reveal a one-dimensional
bound state that runs parallel to the stepedge and is bound to it at some
characteristic distance. This bound state is clearly observed in the bulk gap
region, while it becomes entangled with the oscillations of the warped surface
band at high energy, and with the valence band states near the Dirac point.
Using the full effective Hamiltonian proposed by Zhang et al., we obtain a
closed formula for this bound state that fits the data and provide further
insight into the general topological properties of the electronic structure of
the surface band near strong structural defects.Comment: 5 pages, 4 figure
Charge dynamics of the spin-density-wave state in BaFeAs
We report on a thorough optical investigation of BaFeAs over a broad
spectral range and as a function of temperature, focusing our attention on its
spin-density-wave (SDW) phase transition at K. While
BaFeAs remains metallic at all temperatures, we observe a depletion in
the far infrared energy interval of the optical conductivity below ,
ascribed to the formation of a pseudogap-like feature in the excitation
spectrum. This is accompanied by the narrowing of the Drude term consistent
with the transport results and suggestive of suppression of scattering
channels in the SDW state. About 20% of the spectral weight in the far infrared
energy interval is affected by the SDW phase transition
The influence of magnetic order on the magnetoresistance anisotropy of FeCuTe
We performed resistance measurements on FeCuTe with
in the presence of in-plane applied magnetic fields,
revealing a resistance anisotropy that can be induced at a temperature far
below the structural and magnetic zero-field transition temperatures. The
observed resistance anisotropy strongly depends on the field orientation with
respect to the crystallographic axes, as well as on the field-cooling history.
Our results imply a correlation between the observed features and the
low-temperature magnetic order. Hysteresis in the angle-dependence indicates a
strong pinning of the magnetic order within a temperature range that varies
with the Cu content. The resistance anisotropy vanishes at different
temperatures depending on whether an external magnetic field or a remnant field
is present: the closing temperature is higher in the presence of an external
field. For the resistance anisotropy closes above the
structural transition, at the same temperature at which the zero-field
short-range magnetic order disappears and the sample becomes paramagnetic. Thus
we suggest that under an external magnetic field the resistance anisotropy
mirrors the magnetic order parameter. We discuss similarities to nematic order
observed in other iron pnictide materials.Comment: 11 pages, 9 figure
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