429 research outputs found
Influence of spin fluctuations near the Mott transition: a DMFT study
Dynamics of magnetic moments near the Mott metal-insulator transition is
investigated by a combined slave-rotor and Dynamical Mean-Field Theory solution
of the Hubbard model with additional fully-frustrated random Heisenberg
couplings. In the paramagnetic Mott state, the spinon decomposition allows to
generate a Sachdev-Ye spin liquid in place of the collection of independent
local moments that typically occurs in the absence of magnetic correlations.
Cooling down into the spin-liquid phase, the onset of deviations from pure
Curie behavior in the spin susceptibility is found to be correlated to the
temperature scale at which the Mott transition lines experience a marked
bending. We also demonstrate a weakening of the effective exchange energy upon
approaching the Mott boundary from the Heisenberg limit, due to quantum
fluctuations associated to zero and doubly occupied sites.Comment: 6 pages, 3 figures. V3 was largely expande
Nonparametric instrumental regression with non-convex constraints
This paper considers the nonparametric regression model with an additive
error that is dependent on the explanatory variables. As is common in empirical
studies in epidemiology and economics, it also supposes that valid instrumental
variables are observed. A classical example in microeconomics considers the
consumer demand function as a function of the price of goods and the income,
both variables often considered as endogenous. In this framework, the economic
theory also imposes shape restrictions on the demand function, like
integrability conditions. Motivated by this illustration in microeconomics, we
study an estimator of a nonparametric constrained regression function using
instrumental variables by means of Tikhonov regularization. We derive rates of
convergence for the regularized model both in a deterministic and stochastic
setting under the assumption that the true regression function satisfies a
projected source condition including, because of the non-convexity of the
imposed constraints, an additional smallness condition
Mott transitions in correlated electron systems with orbital degrees of freedom
Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard
model are studied by means of a generalization of the linearized dynamical
mean-field theory. The method allows for an efficient and reliable
determination of the critical interaction U_c for any integer filling n and
different M at zero temperature. For half-filling a linear dependence of U_c on
M is found. Inclusion of the (full) Hund's rule exchange J results in a strong
reduction of U_c. The transition turns out to change qualitatively from
continuous for J=0 to discontinuous for any finite J
Quantum impurity solvers using a slave rotor representation
We introduce a representation of electron operators as a product of a
spin-carry ing fermion and of a phase variable dual to the total charge (slave
quantum rotor). Based on this representation, a new method is proposed for
solving multi-orbital Anderson quantum impurity models at finite interaction
strength U. It consists in a set of coupled integral equations for the
auxiliary field Green's functions, which can be derived from a controlled
saddle-point in the limit of a large number of field components. In contrast to
some finite-U extensions of the non-crossing approximation, the new method
provides a smooth interpolation between the atomic limit and the weak-coupling
limit, and does not display violation of causality at low-frequency. We
demonstrate that this impurity solver can be applied in the context of
Dynamical Mean-Field Theory, at or close to half-filling. Good agreement with
established results on the Mott transition is found, and large values of the
orbital degeneracy can be investigated at low computational cost.Comment: 18 pages, 15 figure
INTS3 controls the hSSB1-mediated DNA damage response
MISE is identified as a component of the Integrator complex required for DNA repair
Robust Nodal Structure of Landau Level Wave Functions Revealed by Fourier Transform Scanning Tunneling Spectroscopy
Scanning tunneling spectroscopy is used to study the real-space local density
of states (LDOS) of a two-dimensional electron system in magnetic field, in
particular within higher Landau levels (LL). By Fourier transforming the LDOS,
we find a set of n radial minima at fixed momenta for the nth LL. The momenta
of the minima depend only on the inverse magnetic length. By comparison with
analytical theory and numerical simulations, we attribute the minima to the
nodes of the quantum cyclotron orbits, which decouple in Fourier representation
from the random guiding center motion due to the disorder. This robustness of
the nodal structure of LL wave functions should be viewed as a key property of
quantum Hall states
Spectroscopic signatures of a bandwidth-controlled Mott transition at the surface of 1T-TaSe
High-resolution angle-resolved photoemission (ARPES) data show that a
metal-insulator Mott transition occurs at the surface of the quasi-two
dimensional compound TaSe. The transition is driven by the narrowing of the
Ta band induced by a temperature-dependent modulation of the atomic
positions. A dynamical mean-field theory calculation of the spectral function
of the half-filled Hubbard model captures the main qualitative feature of the
data, namely the rapid transfer of spectral weight from the observed
quasiparticle peak at the Fermi surface to the Hubbard bands, as the
correlation gap opens up.Comment: 4 pages, 4 figures; one modified figure, added referenc
Mott transition at large orbital degeneracy: dynamical mean-field theory
We study analytically the Mott transition of the N-orbital Hubbard model
using dynamical mean-field theory and a low-energy projection onto an effective
Kondo model. It is demonstrated that the critical interaction at which the
insulator appears (Uc1) and the one at which the metal becomes unstable (Uc2)
have different dependence on the number of orbitals as the latter becomes
large: Uc1 ~ \sqrt{N} while Uc2 ~ N. An exact analytical determination of the
critical coupling Uc2/N is obtained in the large-N limit. The metallic solution
close to this critical coupling has many similarities at low-energy with the
results of slave boson approximations, to which a comparison is made. We also
discuss how the critical temperature associated with the Mott critical endpoint
depends on the number of orbitals.Comment: 13 pages. Minor changes in V
The Membrane-Associated Proteins FCHo and SGIP Are Allosteric Activators of the AP2 Clathrin Adaptor Complex
The AP2 clathrin adaptor complex links protein cargo to the endocytic machinery but it is unclear how AP2 is activated on the plasma membrane. Here we demonstrate that the membrane-associated proteins FCHo and SGIP1 convert AP2 into an open, active conformation. We screened for C. elegans mutants that phenocopy the loss of AP2 subunits and found that AP2 remains inactive in fcho-1 mutants. A subsequent screen for bypass suppressors of fcho-1 nulls identified 71 compensatory mutations in all four AP2 subunits. Using a protease-sensitivity assay we show that these mutations restore the open conformation in vivo. The domain of FCHo that induces this rearrangement is not the F-BAR domain or the mu-homology domain, but rather is an uncharacterized 90 amino acid motif, found in both FCHo and SGIP proteins, that directly binds AP2. Thus, these proteins stabilize nascent endocytic pits by exposing membrane and cargo binding sites on AP2
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