798 research outputs found
Spinon confinement in a quasi one dimensional anisotropic Heisenberg magnet
Confinement is a process by which particles with fractional quantum numbers
bind together to form quasiparticles with integer quantum numbers. The
constituent particles are confined by an attractive interaction whose strength
increases with increasing particle separation and as a consequence, individual
particles are not found in isolation. This phenomenon is well known in particle
physics where quarks are confined in baryons and mesons. An analogous
phenomenon occurs in certain magnetic insulators; weakly coupled chains of
spins S=1/2. The collective excitations in these systems is spinons (S=1/2). At
low temperatures weak coupling between chains can induce an attractive
interaction between pairs of spinons that increases with their separation and
thus leads to confinement. In this paper, we employ inelastic neutron
scattering to investigate the spinon confinement in the quasi-1D S=1/2 XXZ
antiferromagnet SrCo2V2O8. Spinon excitations are observed above TN in
quantitative agreement with established theory. Below TN the pairs of spinons
are confined and two sequences of meson-like bound states with longitudinal and
transverse polarizations are observed. Several theoretical approaches are used
to explain the data. A new theoretical technique based on Tangent-space Matrix
Product States gives a very complete description of the data and provides good
agreement not only with the energies of the bound modes but also with their
intensities. We also successfully explained the effect of temperature on the
excitations including the experimentally observed thermally induced resonance
between longitudinal modes below TN ,and the transitions between thermally
excited spinon states above TN. In summary, our work establishes SrCo2V2O8 as a
beautiful paradigm for spinon confinement in a quasi-1D quantum magnet and
provides a comprehensive picture of this process.Comment: 17 pages, 18 figures, submitted to PR
A Novel Longitudinal Mode in the Coupled Quantum Chain Compound KCuF3
Inelastic neutron scattering measurements are reported that show a new
longitudinal mode in the antiferromagnetically ordered phase of the spin-1/2
quasi-one-dimensional antiferromagnet KCuF3. This mode signals the cross-over
from one-dimensional to three-dimensional behavior and indicates a reduction in
the ordered spin moment of a spin-1/2 antiferromagnet. The measurements are
compared with recent quantum field theory results and are found to be in
excellent agreement. A feature of the data not predicted by theory is a damping
of the mode by decay processes to the transverse spin-wave branches.Comment: 9 pages of text plus 4 postscript figures (1 color
Quantum spin chain as a potential realization of the Nersesyan-Tsvelik model
It is well established that long-range magnetic order is suppressed in
magnetic systems whose interactions are low-dimensional. The prototypical
example is the S-1/2 Heisenberg antiferromagnetic chain (S-1/2 HAFC) whose
ground state is quantum critical. In real S-1/2 HAFC compounds interchain
coupling induces long-range magnetic order although with a suppressed ordered
moment and reduced N\'eel temperature compared to the Curie-Weiss temperature.
Recently, it was suggested that order can also be suppressed if the interchain
interactions are frustrated, as for the Nersesyan-Tsvelik model. Here, we study
the new S-1/2 HAFC, (NO)[Cu(NO3)3]. This material shows extreme suppression of
order which furthermore is incommensurate revealing the presence of frustration
consistent with the Nersesyan-Tsvelik model
Collapse Dynamics of a Star of Dark Matter and Dark Energy
In this work, we study the collapse dynamics of an inhomogeneous spherically
symmetric star made of dark matter (DM) and dark energy (DE). The dark matter
is taken in the form of a dust cloud while anisotropic fluid is chosen as the
candidate for dark energy. It is investigated how dark energy modifies the
collapsing process and is examined whether dark energy has any effect on the
Cosmic Censorship Conjecture. The collapsing star is assumed to be of finite
radius and the space time is divided into three distinct regions and
, where represents the boundary of the star and
denotes the interior (exterior) of the star. The junction
conditions for matching over are specified. Role of Dark
energy in the formation of apparent horizon is studied and central singularity
is analyzed.Comment: 13 page
Cauchy horizon singularity without mass inflation
A perturbed Reissner-Nordstr\"om-de Sitter solution is used to emphasize the
nature of the singularity along the Cauchy horizon of a charged spherically
symmetric black hole. For these solutions, conditions may prevail under which
the mass function is bounded and yet the curvature scalar
diverges.Comment: typeset in RevTex, 13 page
Gravitational Collapse: Expanding and Collapsing Regions
We investigate the expanding and collapsing regions by taking two well-known
spherically symmetric spacetimes. For this purpose, the general formalism is
developed by using Israel junction conditions for arbitrary spacetimes. This
has been used to obtain the surface energy density and the tangential pressure.
The minimal pressure provides the gateway to explore the expanding and
collapsing regions. We take Minkowski and Kantowski-Sachs spacetimes and use
the general formulation to investigate the expanding and collapsing regions of
the shell.Comment: 12 pages, 4 figures, accepted for publication in Gen. Relativ. Gra
Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism
This article reviews the application of the non-equilibrium Green's function
formalism to the simulation of novel photovoltaic devices utilizing quantum
confinement effects in low dimensional absorber structures. It covers
well-known aspects of the fundamental NEGF theory for a system of interacting
electrons, photons and phonons with relevance for the simulation of
optoelectronic devices and introduces at the same time new approaches to the
theoretical description of the elementary processes of photovoltaic device
operation, such as photogeneration via coherent excitonic absorption,
phonon-mediated indirect optical transitions or non-radiative recombination via
defect states. While the description of the theoretical framework is kept as
general as possible, two specific prototypical quantum photovoltaic devices, a
single quantum well photodiode and a silicon-oxide based superlattice absorber,
are used to illustrated the kind of unique insight that numerical simulations
based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape
Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O
Magnetic excitations have been investigated in the spin dimer material
VODPO_4 \cdot 1/2 D_2O using inelastic neutron scattering. A dispersionless
magnetic mode was observed at an energy of 7.81(4) meV. The wavevector
dependence of the scattering intensityfrom this mode is consistent with the
excitation of isolated V^{4+} spin dimers with a V-V separation of 4.43(7) \AA.
This result is unexpected since the V-V pair previously thought to constitute
themagnetic dimer has a separation of 3.09 \AA. We identify an alternative V-V
pair as the likely magnetic dimer, which involves superexchange pathways
through a covalently bonded PO_4 group. This surprising result casts doubt on
the interpretation of (VO)_2P_2O_7 as a spin ladder.Comment: 4 pages, 4 postscript figures - identical to previous paper but
figure 2 and 3 hopefully more compatible .p
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