48,831 research outputs found
Enabling Privacy-preserving Auctions in Big Data
We study how to enable auctions in the big data context to solve many
upcoming data-based decision problems in the near future. We consider the
characteristics of the big data including, but not limited to, velocity,
volume, variety, and veracity, and we believe any auction mechanism design in
the future should take the following factors into consideration: 1) generality
(variety); 2) efficiency and scalability (velocity and volume); 3) truthfulness
and verifiability (veracity). In this paper, we propose a privacy-preserving
construction for auction mechanism design in the big data, which prevents
adversaries from learning unnecessary information except those implied in the
valid output of the auction. More specifically, we considered one of the most
general form of the auction (to deal with the variety), and greatly improved
the the efficiency and scalability by approximating the NP-hard problems and
avoiding the design based on garbled circuits (to deal with velocity and
volume), and finally prevented stakeholders from lying to each other for their
own benefit (to deal with the veracity). We achieve these by introducing a
novel privacy-preserving winner determination algorithm and a novel payment
mechanism. Additionally, we further employ a blind signature scheme as a
building block to let bidders verify the authenticity of their payment reported
by the auctioneer. The comparison with peer work shows that we improve the
asymptotic performance of peer works' overhead from the exponential growth to a
linear growth and from linear growth to a logarithmic growth, which greatly
improves the scalability
Landau level states on a topological insulator thin film
We analyze the four-dimensional Hamiltonian proposed to describe the band
structure of the single-Dirac-cone family of topological insulators in the
presence of a uniform perpendicular magnetic field. Surface Landau level(LL)
states appear, decoupled from the bulk levels and following the quantized
energy dispersion of a purely two-dimensional surface Dirac Hamiltonian. A
small hybridization gap splits the degeneracy of the central n=0 LL with
dependence on the film thickness and the field strength that can be obtained
analytically. Explicit calculation of the spin and charge densities show that
surface LL states are localized within approximately one quintuple layer from
the surface termination. Some new surface-bound LLs are shown to exist at a
higher Landau level index.Comment: 8 pages, 4 figure
Search Me If You Can: Privacy-preserving Location Query Service
Location-Based Service (LBS) becomes increasingly popular with the dramatic
growth of smartphones and social network services (SNS), and its context-rich
functionalities attract considerable users. Many LBS providers use users'
location information to offer them convenience and useful functions. However,
the LBS could greatly breach personal privacy because location itself contains
much information. Hence, preserving location privacy while achieving utility
from it is still an challenging question now. This paper tackles this
non-trivial challenge by designing a suite of novel fine-grained
Privacy-preserving Location Query Protocol (PLQP). Our protocol allows
different levels of location query on encrypted location information for
different users, and it is efficient enough to be applied in mobile platforms.Comment: 9 pages, 1 figure, 2 tables, IEEE INFOCOM 201
Classification of flat bands according to the band-crossing singularity of Bloch wave functions
We show that flat bands can be categorized into two distinct classes, that
is, singular and nonsingular flat bands, by exploiting the singular behavior of
their Bloch wave functions in momentum space. In the case of a singular flat
band, its Bloch wave function possesses immovable discontinuities generated by
the band-crossing with other bands, and thus the vector bundle associated with
the flat band cannot be defined. This singularity precludes the compact
localized states from forming a complete set spanning the flat band. Once the
degeneracy at the band crossing point is lifted, the singular flat band becomes
dispersive and can acquire a finite Chern number in general, suggesting a new
route for obtaining a nearly flat Chern band. On the other hand, the Bloch wave
function of a nonsingular flat band has no singularity, and thus forms a vector
bundle. A nonsingular flat band can be completely isolated from other bands
while preserving the perfect flatness. All one-dimensional flat bands belong to
the nonsingular class. We show that a singular flat band displays a novel
bulk-boundary correspondence such that the presence of the robust boundary mode
is guaranteed by the singularity of the Bloch wave function. Moreover, we
develop a general scheme to construct a flat band model Hamiltonian in which
one can freely design its singular or nonsingular nature. Finally, we propose a
general formula for the compact localized state spanning the flat band, which
can be easily implemented in numerics and offer a basis set useful in analyzing
correlation effects in flat bands.Comment: 23 pages, 13 figure
Searching for topological density wave insulators in multi-orbital square lattice systems
We study topological properties of density wave states with broken
translational symmetry in two-dimensional multi-orbital systems with a
particular focus on t orbitals in square lattice. Due to distinct
symmetry properties of d-orbitals, a nodal charge or spin density wave state
with Dirac points protected by lattice symmetries can be achieved. When an
additional order parameter with opposite reflection symmetry is introduced to a
nodal density wave state, the system can be fully gapped leading to a band
insulator. Among those, topological density wave (TDW) insulators can be
realized, when an effective staggered on-site potential generates a gap to a
pair of Dirac points connected by the inversion symmetry which have the same
topological winding numbers. We also present a mean-field phase diagram for
various density wave states, and discuss experimental implications of our
results.Comment: 15 pages, 10 figures, 7 table
Failure of Nielsen-Ninomiya theorem and fragile topology in two-dimensional systems with space-time inversion symmetry: application to twisted bilayer graphene at magic angle
We show that the Wannier obstruction and the fragile topology of the nearly
flat bands in twisted bilayer graphene at magic angle are manifestations of the
nontrivial topology of two-dimensional real wave functions characterized by the
Euler class. To prove this, we examine the generic band topology of two
dimensional real fermions in systems with space-time inversion
symmetry. The Euler class is an integer topological invariant classifying real
two band systems. We show that a two-band system with a nonzero Euler class
cannot have an -symmetric Wannier representation. Moreover, a two-band
system with the Euler class has band crossing points whose total
winding number is equal to . Thus the conventional Nielsen-Ninomiya
theorem fails in systems with a nonzero Euler class. We propose that the
topological phase transition between two insulators carrying distinct Euler
classes can be described in terms of the pair creation and annihilation of
vortices accompanied by winding number changes across Dirac strings. When the
number of bands is bigger than two, there is a topological invariant
classifying the band topology, that is, the second Stiefel Whitney class
(). Two bands with an even (odd) Euler class turn into a system with
() when additional trivial bands are added. Although the
nontrivial second Stiefel-Whitney class remains robust against adding trivial
bands, it does not impose a Wannier obstruction when the number of bands is
bigger than two. However, when the resulting multi-band system with the
nontrivial second Stiefel-Whitney class is supplemented by additional chiral
symmetry, a nontrivial second-order topology and the associated corner charges
are guaranteed.Comment: 23 pages, 13 figure
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