7,756 research outputs found
Investigation of the [1,5]-hydride shift as a route to nitro-Mannich cyclisations
Conditions were found for the [1,5]-hydride shift nitro-Mannich reaction that led to the synthesis of 2,3-disubstituted tetrahydroquinolines. Two simple cyclic amine substrates gave diastereomerically pure rearranged products in 65 and 90% yields by refluxing in HFIP. A more general procedure used Gd(OTf)3 as a catalyst and successfully rearranged other cyclic and acyclic amines in 42–84% yield with diastereomeric ratios of 75:25 to >95:5 in favour of the anti-diastereoisomer (9 examples). Two examples of sulphur containing heterocycles gave lower yields of 9 and 25%. Electron withdrawing substituents were shown to have a deleterious effect on the success of the reaction. The results indicated the limitation of the [1,5]-hydride shift nitro-Mannich reaction with respect to the stability of the intermediate iminium ion
Instability of a Landau Fermi liquid as the Mott insulator is approached
We examine a two-dimensional Fermi liquid with a Fermi surface which touches
the Umklapp surface first at the 4 points as the
electron density is increased. Umklapp processes at the 4 patches near lead the renormalization group equations to scale to strong
coupling resembling the behavior of a 2-leg ladder at half-filling. The
incompressible character of the fixed point causes a breakdown of Landau theory
at these patches. A further increase in density spreads the incompressible
regions so that the open Fermi surface shrinks to 4 disconnected segments. This
non-Landau state, in which parts of the Fermi surface are truncated to form an
insulating spin liquid, has many features in common with phenomenological
models recently proposed for the cuprate superconductors.Comment: Minor changes. LaTeX2e, 12 pages, 5 figures. J. Phys. CM 10 (1998)
L38
Anisotropy of Magnetic Interactions in the Spin-Ladder Compound (CHN)CuBr
Magnetic excitations in the spin-ladder material (CHN)CuBr
[BPCB] are probed by high-resolution multi-frequency electron spin resonance
(ESR) spectroscopy. Our experiments provide a direct evidence for a biaxial
anisotropy ( of the dominant exchange interaction), that is in
contrast to a fully isotropic spin-ladder model employed for this system
previously. It is argued that this anisotropy in BPCB is caused by spin-orbit
coupling, which appears to be important for describing magnetic properties of
this compound. The zero-field zone-center gap in the excitation spectrum of
BPCB, K, is detected directly. Furthermore, an ESR
signature of the inter-ladder exchange interactions is obtained. The detailed
characterization of the anisotropy in BPCB completes the determination of the
full spin hamiltonian of this exceptional spin-ladder material and shows ways
to study anisotropy effects in spin ladders.Comment: 6 pages, 6 figure
A Rigorous Proof of Fermi Liquid Behavior for Jellium Two-Dimensional Interacting Fermions
Using the method of continuous constructive renormalization group around the
Fermi surface, it is proved that a jellium two-dimensional interacting system
of Fermions at low temperature remains analytic in the coupling constant
for where is some numerical constant
and is the temperature. Furthermore in that range of parameters, the first
and second derivatives of the self-energy remain bounded, a behavior which is
that of Fermi liquids and in particular excludes Luttinger liquid behavior. Our
results prove also that in dimension two any transition temperature must be
non-perturbative in the coupling constant, a result expected on physical
grounds. The proof exploits the specific momentum conservation rules in two
dimensions.Comment: 4 pages, no figure
Numerical Jordan-Wigner approach for two dimensional spin systems
We present a numerical self consistent variational approach based on the
Jordan-Wigner transformation for two dimensional spin systems. We apply it to
the study of the well known quantum (S=1/2) antiferromagnetic XXZ system as a
function of the easy-axis anisotropy \Delta on a periodic square lattice. For
the SU(2) case the method converges to a N\'eel ordered ground state
irrespectively of the input density profile used and in accordance with other
studies. This shows the potential utility of the proposed method to investigate
more complicated situations like frustrated or disordered systems.Comment: Revtex, 8 pages, 4 figure
Metal-insulator crossover in the Boson-Fermion model in infinite dimensions
The Boson-Fermion model, describing a mixture of tightly bound electron pairs
and quasi-free electrons hybridized with each other via a charge exchange term,
is studied in the limit of infinite dimensions, using the Non-Crossing
Approximation within the Dynamical Mean Field Theory. It is shown that a
metal-insulator crossover, driven by strong pair fluctuations, takes place as
the temperature is lowered. It manifests itself in the opening of a pseudogap
in the electron density of states, accompanied by a corresponding effect in the
optical and dc conductivity.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let
High-Field ESR Measurements of S=1/2 Kagome Lattice Antiferromagnet BaCuVO(OH)
High-field electron spin resonance (ESR) measurements have been performed on
vesignieite BaCuVO(OH), which is considered as a nearly ideal
model substance of =1/2 kagome antiferromagnet, in the temperature region
from 1.9 to 265 K. The frequency region is from 60 to 360 GHz and the applied
pulsed magnetic field is up to 16 T. Observed g-value and linewidth show the
increase below 20 K, which suggest the development of the short range order.
Moreover, a gapless spin liquid ground state is suggested from the
frequency-field relation at 1.9 K.Comment: 5 pages, 6 figures, jpsj2 class file, to be published in J. Phys.
Soc. Jp
Measurements in two bases are sufficient for certifying high-dimensional entanglement
High-dimensional encoding of quantum information provides a promising method
of transcending current limitations in quantum communication. One of the
central challenges in the pursuit of such an approach is the certification of
high-dimensional entanglement. In particular, it is desirable to do so without
resorting to inefficient full state tomography. Here, we show how carefully
constructed measurements in two bases (one of which is not orthonormal) can be
used to faithfully and efficiently certify bipartite high-dimensional states
and their entanglement for any physical platform. To showcase the practicality
of this approach under realistic conditions, we put it to the test for photons
entangled in their orbital angular momentum. In our experimental setup, we are
able to verify 9-dimensional entanglement for a pair of photons on a
11-dimensional subspace each, at present the highest amount certified without
any assumptions on the state.Comment: 11+14 pages, 2+7 figure
Spreading, Nonergodicity, and Selftrapping: a puzzle of interacting disordered lattice waves
Localization of waves by disorder is a fundamental physical problem
encompassing a diverse spectrum of theoretical, experimental and numerical
studies in the context of metal-insulator transitions, the quantum Hall effect,
light propagation in photonic crystals, and dynamics of ultra-cold atoms in
optical arrays, to name just a few examples. Large intensity light can induce
nonlinear response, ultracold atomic gases can be tuned into an interacting
regime, which leads again to nonlinear wave equations on a mean field level.
The interplay between disorder and nonlinearity, their localizing and
delocalizing effects is currently an intriguing and challenging issue in the
field of lattice waves. In particular it leads to the prediction and
observation of two different regimes of destruction of Anderson localization -
asymptotic weak chaos, and intermediate strong chaos, separated by a crossover
condition on densities. On the other side approximate full quantum interacting
many body treatments were recently used to predict and obtain a novel many body
localization transition, and two distinct phases - a localization phase, and a
delocalization phase, both again separated by some typical density scale. We
will discuss selftrapping, nonergodicity and nonGibbsean phases which are
typical for such discrete models with particle number conservation and their
relation to the above crossover and transition physics. We will also discuss
potential connections to quantum many body theories.Comment: 13 pages in Springer International Publishing Switzerland 2016 1 M.
Tlidi and M. G. Clerc (eds.), Nonlinear Dynamics: Materials, Theory and
Experiment, Springer Proceedings in Physics 173. arXiv admin note: text
overlap with arXiv:1405.112
Localization from quantum interference in one-dimensional disordered potentials
We show that the tails of the asymptotic density distribution of a quantum
wave packet that localizes in the the presence of random or quasiperiodic
disorder can be described by the diagonal term of the projection over the
eingenstates of the disordered potential. This is equivalent of assuming a
phase randomization of the off-diagonal/interference terms. We demonstrate
these results through numerical calculations of the dynamics of ultracold atoms
in the one-dimensional speckle and quasiperiodic potentials used in the recent
experiments that lead to the observation of Anderson localization for matter
waves [Billy et al., Nature 453, 891 (2008); Roati et al., Nature 453, 895
(2008)]. For the quasiperiodic case, we also discuss the implications of using
continuos or discrete models.Comment: 5 pages, 3 figures; minor changes, references update
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