Optical Conductivity Anisotropy in the Undoped Three-Orbital Hubbard Model for the Pnictides. (arXiv:1109.0012v1 [cond-mat.supr-con])

The resistivity anisotropy unveiled in the study of detwinned single crystals
of the undoped 122 pnictides is here studied using the two-dimensional
three-orbital Hubbard model in the mean-field approximation. Calculating the
Drude weight in the x and y directions at zero temperature for a Q=(\pi,0)
magnetically ordered state, the conductance along the antiferromagnetic
direction is shown to be larger than along the ferromagnetic direction. This
effect is caused by the suppression of the d_{yz} orbital at the Fermi surface,
but additional insight based on the momentum dependence of the transitions
induced by the current operator is provided. It is shown that the effective
suppression of the inter-orbital hopping d_{xy} and d_{yz} along the y
direction is the main cause of the anisotropy.

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Stationary phase approach to the quasiparticle interference on the surface of three dimensional strong topological insulators. (arXiv:1108.6051v1 [cond-mat.mes-hall])

Constant energy contour (CEC) of the surface bands in topological insulators
varies not only with materials but also at different energies. The
quasiparticle interference caused by scattering-off from defects on the surface
of topological insulators is an effective way to reveal the topologies of the
CEC and can be probed by scanning tunneling microscopy (STM). Using stationary
phase approach, a general analytic formulation of the local density of states
as well as the power laws of the Friedel oscillation are present, based only on
the time-reversal symmetry and the local geometry around the scattering end
points on the CEC. Distinct response of surface states to magnetic impurities
from that of nonmagnetic impurities is predicted in particular, which is
proposed to be measured in a closed “magnetic wall” setup on the surface of
topological insulators.

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Driving Force of Ultrafast Magnetization Dynamics. (arXiv:1108.5170v1 [cond-mat.mtrl-sci])

Irradiating a ferromagnetic material with an ultrashort laser pulse leads to
demagnetization on a femtosecond timescale. We implement Elliott-Yafet type
spin-flip scattering, mediated by electron-electron and electron-phonon
collisions, into the framework of a spin-resolved Boltzmann equation.
Considering three mutually coupled reservoirs, (i) spin-up electrons, (ii)
spin-down electrons and (iii) phonons, we trace non-equilibrium electron
distributions during and after laser excitation. We identify the driving force
for ultrafast magnetization dynamics as the equilibration of temperatures and
chemical potentials between the electronic subsystems. This principle can be
used to easily predict the maximum quenching of magnetization upon ultrashort
laser irradiation in any material, as we show for the example of
3d-ferromagnetic nickel.

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Entanglement Entropy of Gapped Phases and Topological Order in Three dimensions. (arXiv:1108.4038v1 [cond-mat.str-el])

We discuss entanglement entropy of gapped ground states in different
dimensions, obtained on partitioning space into two regions. For trivial phases
without topological order, we argue that the entanglement entropy may be
obtained by integrating an `entropy density’ over the partition boundary that
admits a gradient expansion in the curvature of the boundary. This constrains
the expansion of entanglement entropy as a function of system size, and points
to an even-odd dependence on dimensionality. For example, in contrast to the
familiar result in two dimensions, a size independent constant contribution to
the entanglement entropy can appear for trivial phases in any odd spatial
dimension. We then discuss phases with topological entanglement entropy (TEE)
that cannot be obtained by adding local contributions. We find that in three
dimensions there is just one type of TEE, as in two dimensions, that depends
linearly on the number of connected components of the boundary (the `zeroth
Betti number’). In D > 3 dimensions, new types of TEE appear which depend on
the higher Betti numbers of the boundary manifold. We construct generalized
toric code models that exhibit these TEEs and discuss ways to extract TEE in D
>=3.

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Mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and acetonitrile: a molecular simulation. (arXiv:1108.3607v1 [cond-mat.mtrl-sci])

Recently, we introduced a new force field (FF) to simulate transport
properties of imidazolium-based room-temperature ionic liquids (RTILs) using a
solid physical background. In the present work, we apply this FF to derive
thermodynamic, structure, and transport properties of the mixtures of
1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], and acetonitrile
(ACN) over the whole composition range. Three approaches to derive a force
field are formulated based on different treatments of the ion-ion and
ion-molecule Coulomb interactions using unit-charge, scaled-charge and
floating-charge approaches. The simulation results are justified with the help
of experimental data on specific density and shear viscosity for these
mixtures. We find that a phenomenological account (particularly, simple
scaled-charge model) of electronic polarization leads to the best-performing
model. Remarkably, its validity does not depend on the molar fraction of
[BMIM][BF4] in the mixture. The derived FF is so far the first molecular model
which is able to simulate all transport properties of the mixtures, comprising
RTIL and ACN, fully realistically.

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Dissociation of O2 molecules on strained Pb(111) surfaces. (arXiv:1108.3604v1 [cond-mat.mtrl-sci])

By performing first-principles molecular dynamics calculations, we
systematically simulate the adsorption behavior of oxygen molecules on the
clean and strained Pb(111) surfaces. The obtained molecular adsorption
precursor state, and the activated dissociation process for oxygen molecules on
the clean Pb surface are in good agreements with our previous static
calculations, and perfectly explains previous experimental observations [Proc.
Natl. Acad. Sci. U.S.A. 104, 9204 (2007)]. In addition, we also study the
influences of surface strain on the dissociation behaviors of O2 molecules. It
is found that on the compressed Pb(111) surfaces with a strain value of larger
than 0.02, O2 molecules will not dissociate at all. And on the stretched
Pb(111) surfaces, O2 molecules become easier to approach, and the adsorption
energy of the dissociated oxygen atoms is larger than that on the clean Pb
surface.

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Hysteresis and Return Point Memory in Artificial Spin Ice Systems. (arXiv:1108.3584v1 [cond-mat.stat-mech])

We investigate hysteresis loops and return point memory for artificial square
and kagome spin ice systems by cycling an applied bias force and comparing
microscopic effective spin configurations throughout the hysteresis cycle.
Return point memory loss is caused by motion of individual defects in kagome
ice or of grain boundaries in square ice. In successive cycles, return point
memory is recovered rapidly in kagome ice. Memory is recovered more gradually
in square ice due to the extended nature of the grain boundaries. Increasing
the amount of quenched disorder increases the defect density but also enhances
the return point memory since the defects become trapped more easily.

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Singularity in self-energy and composite fermion excitations of interacting electrons. (arXiv:1108.3336v1 [cond-mat.str-el])

We propose that a composite fermion operator $ f_{i\sigma}(2n_{i{\bar
\sigma}}-1)$ could have coherent excitations, where $ f_{i\sigma}$ is the
fermion operator for interacting electrons and $ n_{i{\bar \sigma}}$ is the
number operator of the opposite spin. In the two-impurity Anderson model, it is
found that the excitation of this composite fermion has a pseudogap in the
Kondo regime, and has a finite spectral weight in the regime where the
excitation of the regular fermion $ f_{i\sigma}$ has a pseudogap. In the latter
regime, the self-energy of $ f_{i\sigma}$ is found to be singular near Fermi
energy. We argue that this composite fermion could develop a Fermi surface with
Fermi liquid behaviors but “hidden” from charge excitations in lattice
generalizations. We further illustrate that this type of excitations is
essential in addressing the pseudogap state and unconventional
superconductivity.

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Topological Phase Transitions for Interacting Finite Systems. (arXiv:1108.2507v1 [cond-mat.str-el])

In this Letter, we investigate the signatures of the topological phase
transition in interacting systems. The key signature is the existence of a
topologically protected level crossing, which is robust and sharply defines the
topological transition, even in finite-size systems. We discuss the theoretical
implications of this discovery and utilize exact diagonalization to demonstrate
this signature in the Haldane-Fermi-Hubbard model. This result provides a new,
efficient way to detect topological transitions in experiments and also in
numerical methods that cannot access the ground state wavefunction.

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Phase diagram of the half-filled one-dimensional $t$-$V$-$V’$ model. (arXiv:1108.2272v1 [cond-mat.str-el])

We study the phase diagram of spinless fermions with nearest and
next-nearest-neighbor interactions in one dimension utilizing the (finite-size)
density-matrix renormalization group (DMRG) method. The competition between
nearest and next-nearest-neighbor interactions and nearest-neighbor hopping
generates four phases in this model: two charge-density-wave insulators, a
Luttinger liquid phase, and a bond-order phase. We use finite-size scaling of
the gap and various structure factors to determine the phase diagram.

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