1,259 research outputs found
Area law violations in a supersymmetric model
We study the structure of entanglement in a supersymmetric lattice model of
fermions on certain types of decorated graphs with quenched disorder. In
particular, we construct models with controllable ground state degeneracy
protected by supersymmetry and the choice of Hilbert space. We show that in
certain special limits these degenerate ground states are associated with local
impurities and that there exists a basis of the ground state manifold in which
every basis element satisfies a boundary law for entanglement entropy. On the
other hand, by considering incoherent mixtures or coherent superpositions of
these localized ground states, we can find regions that violate the boundary
law for entanglement entropy over a wide range of length scales. More
generally, we discuss various desiderata for constructing violations of the
boundary law for entanglement entropy and discuss possible relations of our
work to recent holographic studies.Comment: 20 pages, 1 figure, 1 appendi
Functionality in single-molecule devices: Model calculations and applications of the inelastic electron tunneling signal in molecular junctions
We analyze how functionality could be obtained within single-molecule devices
by using a combination of non-equilibrium Green's functions and ab-initio
calculations to study the inelastic transport properties of single-molecule
junctions. First we apply a full non-equilibrium Green's function technique to
a model system with electron-vibration coupling. We show that the features in
the inelastic electron tunneling spectra (IETS) of the molecular junctions are
virtually independent of the nature of the molecule-lead contacts. Since the
contacts are not easily reproducible from one device to another, this is a very
useful property. The IETS signal is much more robust versus modifications at
the contacts and hence can be used to build functional nanodevices. Second, we
consider a realistic model of a organic conjugated molecule. We use ab-initio
calculations to study how the vibronic properties of the molecule can be
controlled by an external electric field which acts as a gate voltage. The
control, through the gate voltage, of the vibron frequencies and (more
importantly) of the electron-vibron coupling enables the construction of
functionality: non-linear amplification and/or switching is obtained from the
IETS signal within a single-molecule device.Comment: Accepted for publication in Journal of Chemical Physic
Topological nature of spinons and holons: Elementary excitations from matrix product states with conserved symmetries
We develop variational matrix product state (MPS) methods with symmetries to
determine dispersion relations of one dimensional quantum lattices as a
function of momentum and preset quantum number. We test our methods on the XXZ
spin chain, the Hubbard model and a non-integrable extended Hubbard model, and
determine the excitation spectra with a precision similar to the one of the
ground state. The formulation in terms of quantum numbers makes the topological
nature of spinons and holons very explicit. In addition, the method also
enables an easy and efficient direct calculation of the necessary magnetic
field or chemical potential required for a certain ground state magnetization
or particle density.Comment: 13 pages, 4 pages appendix, 8 figure
The Algebraic Bethe Ansatz and Tensor Networks
We describe the Algebraic Bethe Ansatz for the spin-1/2 XXX and XXZ
Heisenberg chains with open and periodic boundary conditions in terms of tensor
networks. These Bethe eigenstates have the structure of Matrix Product States
with a conserved number of down-spins. The tensor network formulation suggestes
possible extensions of the Algebraic Bethe Ansatz to two dimensions
Thermal States as Convex Combinations of Matrix Product States
We study thermal states of strongly interacting quantum spin chains and prove
that those can be represented in terms of convex combinations of matrix product
states. Apart from revealing new features of the entanglement structure of
Gibbs states our results provide a theoretical justification for the use of
White's algorithm of minimally entangled typical thermal states. Furthermore,
we shed new light on time dependent matrix product state algorithms which yield
hydrodynamical descriptions of the underlying dynamics.Comment: v3: 10 pages, 2 figures, final published versio
Binegativity and geometry of entangled states in two qubits
We prove that the binegativity is always positive for any two-qubit state. As
a result, as suggested by the previous works, the asymptotic relative entropy
of entanglement in two qubits does not exceed the Rains bound, and the
PPT-entanglement cost for any two-qubit state is determined to be the
logarithmic negativity of the state. Further, the proof reveals some
geometrical characteristics of the entangled states, and shows that the partial
transposition can give another separable approximation of the entangled state
in two qubits.Comment: 5 pages, 3 figures. I made the proof more transparen
Superfluidity of fermions with repulsive on-site interaction in an anisotropic optical lattice near a Feshbach resonance
We present a numerical study on ground state properties of a one-dimensional
(1D) general Hubbard model (GHM) with particle-assisted tunnelling rates and
repulsive on-site interaction (positive-U), which describes fermionic atoms in
an anisotropic optical lattice near a wide Feshbach resonance. For our
calculation, we utilize the time evolving block decimation (TEBD) algorithm,
which is an extension of the density matrix renormalization group and provides
a well-controlled method for 1D systems. We show that the positive-U GHM, when
hole-doped from half-filling, exhibits a phase with coexistence of
quasi-long-range superfluid and charge-density-wave orders. This feature is
different from the property of the conventional Hubbard model with positive-U,
indicating the particle-assisted tunnelling mechanism in GHM brings in
qualitatively new physics.Comment: updated with published version
Aromatic emission from the ionised mane of the Horsehead nebula
We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across
the illuminated edge of the Horsehead nebula and especially their survival and
properties in the HII region. We present spectral mapping observations taken
with the Infrared Spectrograph (IRS) at wavelengths 5.2-38 microns. A strong
AIB at 11.3 microns is detected in the HII region, relative to the other AIBs
at 6.2, 7.7 and 8.6 microns. The intensity of this band appears to be
correlated with the intensity of the [NeII] at 12.8 microns and of Halpha,
which shows that the emitters of the 11.3 microns band are located in the
ionised gas. The survival of PAHs in the HII region could be due to the
moderate intensity of the radiation field (G0 about 100) and the lack of
photons with energy above about 25eV. The enhancement of the intensity of the
11.3 microns band in the HII region, relative to the other AIBs can be
explained by the presence of neutral PAHs. Our observations highlight a
transition region between ionised and neutral PAHs observed with ideal
conditions in our Galaxy. A scenario where PAHs can survive in HII regions and
be significantly neutral could explain the detection of a prominent 11.3
microns band in other Spitzer observations.Comment: 9 pages, 9 figures, accepted for publication in A&
- …