604 research outputs found
Kondo resonance line-shape of magnetic adatoms on decoupling layers
The zero-bias resonance in the dI/dV tunneling spectrum recorded using a
scanning tunneling microscope above a spin-1/2 magnetic adatom (such as Ti)
adsorbed on a decoupling layer on metal surface can be accurately fitted using
the universal spectral function of the Kondo impurity model both at zero field
and at finite external magnetic field. Excellent agreement is found both for
the asymptotic low-energy part and for the high-energy logarithmic tails of the
Kondo resonance. For finite magnetic field, the nonlinear fitting procedure
consists in repeatedly solving the impurity model for different Zeeman energies
in order to obtain accurate spectral functions which are compared with the
experimental dI/dV curves. The experimental results at zero field are
sufficiently restraining to enable an unprecedented reliability in the
determination of the Kondo temperature, while at finite fields the results are
more ambiguous and two different interpretations are proposed
Magnetic anisotropy effects on quantum impurities in superconducting host
We study the magnetic anisotropy effects on the localized sub-gap excitations
induced by quantum impurities coupled to a superconducting host. We establish
the ground-state phase diagrams for single-channel and two-channel high-spin
Kondo impurities; they unveil surprising complexity that results from the
(multi-stage) Kondo screening in competition with the superconducting
correlations and the magnetic anisotropy splitting of the spin multiplets. We
discuss the possibility of detecting the Zeeman splitting of the sub-gap
states, which would provide an interesting spectroscopic tool for studying the
magnetism on the single-atom level. We also study the problem of two impurities
coupled by the Heisenberg exchange interaction, and we follow the evolution of
the sub-gap states for both antiferromagnetic and ferromagnetic coupling. For
sufficiently strong antiferromagnetic coupling, the impurities bind into a
singlet state that is non-magnetic, thus the sub-gap states move to the edge of
the gap and can no longer be discerned. For ferromagnetic coupling, some
excited states remain present inside the gap.Comment: 14 pages, 16 figures (significantly extended version, includes a
section on the two-impurity effects
Fano-Kondo effect in side-coupled double quantum dots at finite temperatures and the importance of the two-stage Kondo screening
We study the zero-bias conductance through the system of two quantum dots,
one of which is embedded directly between the source and drain electrodes,
while the second dot is side-coupled to the first one through a tunneling
junction. Modeling the system using the two-impurity Anderson model, we compute
the temperature-dependence of the conductance in various parameter regimes
using the numerical renormalization group. We consider the non-interacting
case, where we study the extent of the departure from the conventional Fano
resonance line shape at finite temperatures, and the case where the embedded
and/or the side-coupled quantum dot is interacting, where we study the
consequences of the coexistence of the Kondo and Fano effects. If the
side-coupled dot is very weakly interacting, the occupancy changes by two when
the on-site energy crosses the Fermi level and a Fano-resonance-like shape is
observed. If the interaction on the side-coupled dot is sizeable, the occupancy
changes only by one and a very different line-shape results, which is strongly
and characteristically temperature dependent. These results suggest an
intriguing alternative interpretation of the recent experimental results study
of the transport properties of the side-coupled double quantum dot [Sasaki et
al., Phys. Rev. Lett. 103, 266806 (2009)]: the observed Fano-like conductance
anti-resonance may, in fact, result from the two-stage Kondo effect in the
regime where the experimental temperature is between the higher and the lower
Kondo temperature.Comment: 9 pages, 11 figures. In V2: updated references, 3 new figures,
additional discussio
Convergence acceleration and stabilization for dynamical-mean-field-theory calculations
The convergence to the self-consistency in the dynamical-mean-field-theory
(DMFT) calculations for models of correlated electron systems can be
significantly accelerated by using an appropriate mixing of hybridization
functions which are used as the input to the impurity solver. It is shown that
the techniques and the past experience with the mixing of input charge
densities in the density-functional-theory (DFT) calculations are also
effective in DMFT. As an example, the increase of the computational
requirements near the Mott metal-insulator transition in the Hubbard model due
to critical slowing down can be strongly reduced by using the modified
Broyden's method to numerically solve the non-linear self-consistency equation.
Speed-up factors as high as 3 were observed in practical calculations even for
this relatively well behaved problem. Furthermore, the convergence can be
achieved in difficult cases where simple linear mixing is either not effective
or even leads to divergence. Unstable and metastable solutions can also be
obtained. We also determine the linear response of the system with respect to
the variations of the hybridization function, which is related to the
propagation of the information between the different energy scales during the
iteration.Comment: 9 pages, 8 figure
Spin-charge separation and simultaneous spin and charge Kondo effect
We study the spin-charge separation in a Kondo-like model for an impurity
with a spin and a charge (isospin) degree of freedom coupled to a single
conduction channel (the ``spin-charge'' Kondo model). We show that the spin and
charge Kondo effects can occur simultaneously at any coupling strength. In the
continuum (wide-band or weak coupling) limit, the Kondo screening in each
sector is independent, while at finite bandwidth and strong coupling the
lattice effects lead to a renormalization of the effective Kondo exchange
constants; nevertheless, universal spin and charge Kondo effects still occur.
We find similar behavior in the two-impurity Anderson model with positive and
negative electron-electron interaction and in the two-impurity
Anderson-Holstein model with a single phonon mode. We comment on the
applicability of such models to describe the conductance of deformable
molecules with a local magnetic moment.Comment: 13 pages, 11 figure
Integrability of a globally coupled complex Riccati array: quadratic integrate-and-fire neurons, phase oscillators and all in between
We present an exact dimensionality reduction for dynamics of an arbitrary
array of globally coupled complex-valued Riccati equations. It generalizes the
Watanabe-Strogatz theory [Phys. Rev. Lett. 70, 2391 (1993)] for sinusoidally
coupled phase oscillators and seamlessly includes quadratic integrate-and-fire
neurons represented by the special case of real-valued Riccati equations. It
provides a low dimensional description to a wide new class of complex dynamical
systems, warranting their rigorous analysis and thus providing deep insights
into their collective dynamics. This result represents a significant
advancement in our comprehending of coupled oscillatory systems and opens up
many new avenues of research.Comment: 6 pages, 4 figure
Numerical renormalization group study of two-channel three-impurity triangular clusters
We study triangular clusters of three spin-1/2 Kondo or Anderson impurities
that are coupled to two conduction leads. In the case of Kondo impurities, the
model takes the form of an antiferromagnetic Heisenberg ring with Kondo-like
exchange coupling to continuum electrons. We show that this model exhibits many
types of the behavior found in various simpler one and two-impurity models,
thereby enabling the study of crossovers between a number of Fermi-liquid (FL)
and non-Fermi-liquid (NFL) fixed points. In particular, we explore a direct
crossover between the two-impurity Kondo-model NFL fixed point and the
two-channel Kondo-model NFL fixed point. We show that the concept of the
two-stage Kondo effect applies even in the case when the first-stage Kondo
state is of NFL type. In the case of Anderson impurities, we consider the
transport properties of three coupled quantum dots. This class of models
includes as limiting cases the familiar serial double quantum dot and triple
quantum dot nanostructures. By extracting the quasiparticle scattering phase
shifts, we compute the low-temperature conductance as a function of the
inter-impurity tunneling-coupling. We point out that due to the existence of
exponentially low temperature scales, there is a parameter range where the
stable "zero-temperature" fixed point is essentially never reached (not even in
numerical renormalization group calculations). The "zero-temperature"
conductance is then of no interest and it may only be meaningful to compute the
conductance at finite temperature. This illustrates the perils of studying the
conductance in the ground state and considering thermal fluctuations only as a
small correction.Comment: 12 pages, 13 figure
Magnetoresistive biosensors with on-chip pulsed excitation and magnetic correlated double sampling.
Giant magnetoresistive (GMR) sensors have been shown to be among the most sensitive biosensors reported. While high-density and scalable sensor arrays are desirable for achieving multiplex detection, scalability remains challenging because of long data acquisition time using conventional readout methods. In this paper, we present a scalable magnetoresistive biosensor array with an on-chip magnetic field generator and a high-speed data acquisition method. The on-chip field generators enable magnetic correlated double sampling (MCDS) and global chopper stabilization to suppress 1/f noise and offset. A measurement with the proposed system takes only 20 ms, approximately 50× faster than conventional frequency domain analysis. A corresponding time domain temperature correction technique is also presented and shown to be able to remove temperature dependence from the measured signal without extra measurements or reference sensors. Measurements demonstrate detection of magnetic nanoparticles (MNPs) at a signal level as low as 6.92 ppm. The small form factor enables the proposed platform to be portable as well as having high sensitivity and rapid readout, desirable features for next generation diagnostic systems, especially in point-of-care (POC) settings
Imprints of radial migration on the Milky Way’s metallicity distribution functions
Recent analysis of the SDSS-III/Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 12 stellar catalog has revealed that the Milky Way’s (MW) metallicity distribution function (MDF) changes shape as a function of radius, transitioning from being negatively skewed at small Galactocentric radii to positively skewed at large Galactocentric radii. Using a high-resolution, N-body+SPH simulation, we show that the changing skewness arises from radial migration—metal-rich stars form in the inner disk and subsequently migrate to the metal-poorer outer disk. These migrated stars represent a large fraction (>50%) of the stars in the outer disk; they populate the high-metallicity tail of the MDFs and are, in general, more metal-rich than the surrounding outer disk gas. The simulation also reproduces another surprising APOGEE result: the spatially invariant high-[α/Fe] MDFs. This arises in the simulation from the migration of a population formed within a narrow range of radii (3.2 ±1.2 kpc) and time (8.8 ± 0.6 Gyr ago), rather than from spatially extended star formation in a homogeneous medium at early times. These results point toward the crucial role radial migration has played in shaping our MW
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