49 research outputs found
Evidence for localization and 0.7 anomaly in hole quantum point contacts
Quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are
investigated by low-temperature electrical conductance spectroscopy
measurements. Besides one-dimensional conductance quantization in units of
a pronounced extra plateau is found at about which
possesses the characteristic properties of the so-called "0.7 anomaly" known
from experiments with n-type samples. The evolution of the 0.7 plateau in high
perpendicular magnetic field reveals the existence of a quasi-localized state
and supports the explanation of the 0.7 anomaly based on self-consistent charge
localization. These observations are robust when lateral electrical fields are
applied which shift the relative position of the electron wavefunction in the
quantum point contact, testifying to the intrinsic nature of the underlying
physics.Comment: 4.2 pages, 3 figure
Origins of conductance anomalies in a p-type GaAs quantum point contact
Low temperature transport measurements on a p-GaAs quantum point contact are
presented which reveal the presence of a conductance anomaly that is markedly
different from the conventional `0.7 anomaly'. A lateral shift by asymmetric
gating of the conducting channel is utilized to identify and separate different
conductance anomalies of local and generic origins experimentally. While the
more generic 0.7 anomaly is not directly affected by changing the gate
configuration, a model is proposed which attributes the additional conductance
features to a gate-dependent coupling of the propagating states to localized
states emerging due to a nearby potential imperfection. Finite bias
conductivity measurements reveal the interplay between the two anomalies
consistently with a two-impurity Kondo model
Observation of excited states in a p-type GaAs quantum dot
A quantum dot fabricated by scanning probe oxidation lithography on a p-type,
C-doped GaAs/AlGaAs heterostructure is investigated by low temperature
electrical conductance measurements. Clear Coulomb blockade oscillations are
observed and analyzed in terms of sequential tunneling through the
single-particle levels of the dot at T_hole = 185 mK. The charging energies as
large as 2 meV evaluated from Coulomb diamond measurements together with the
well resolved single-hole excited state lines in the charge stability diagram
indicate that the dot is operated with a small number of confined particles
close to the ultimate single-hole regime.Comment: 5 pages, 5 figure
decay form factors from three-flavor lattice QCD
We compute the form factors for the semileptonic decay
process in lattice QCD using gauge-field ensembles with 2+1 flavors of sea
quark, generated by the MILC Collaboration. The ensembles span lattice spacings
from 0.12 to 0.045 fm and have multiple sea-quark masses to help control the
chiral extrapolation. The asqtad improved staggered action is used for the
light valence and sea quarks, and the clover action with the Fermilab
interpretation is used for the heavy quark. We present results for the form
factors , , and , where is the momentum
transfer, together with a comprehensive examination of systematic errors.
Lattice QCD determines the form factors for a limited range of , and we
use the model-independent expansion to cover the whole kinematically
allowed range. We present our final form-factor results as coefficients of the
expansion and the correlations between them, where the errors on the
coefficients include statistical and all systematic uncertainties. We use this
complete description of the form factors to test QCD predictions of the form
factors at high and low . We also compare a Standard-Model calculation of
the branching ratio for with experimental data.Comment: V2: Fig.7 added. Typos text corrected. Reference added. Version
published in Phys. Rev.
A Scaling Approach for Interacting Quantum Wires -a Possible Explanation for the 0.7 Anomalous Conductance
We consider a weakly interacting finite wire with short and long range
interactions. The long range interactions enhance the scattering and
renormalize the wire to a strongly interacting limit. For large screening
lengths, the renormalized charge stiffness Luttinger parameter
decreases to , giving rise to a Wigner crystal at T=0 with an
anomalous conductance at finite temperatures.
For short screening lengths, the renormalized Luttinger parameter
is restricted to . As a result, at temperatures
larger than the magnetic exchange energy we find an interacting metal which for
is equivalent to the Hubbard model, with
the anomalous conductance .Comment: 26 pages and 6 figure
Extreme sensitivity of the spin-splitting and 0.7 anomaly to confining potential in one-dimensional nanoelectronic devices
Quantum point contacts (QPCs) have shown promise as nanoscale spin-selective
components for spintronic applications and are of fundamental interest in the
study of electron many-body effects such as the 0.7 x 2e^2/h anomaly. We report
on the dependence of the 1D Lande g-factor g* and 0.7 anomaly on electron
density and confinement in QPCs with two different top-gate architectures. We
obtain g* values up to 2.8 for the lowest 1D subband, significantly exceeding
previous in-plane g-factor values in AlGaAs/GaAs QPCs, and approaching that in
InGaAs/InP QPCs. We show that g* is highly sensitive to confinement potential,
particularly for the lowest 1D subband. This suggests careful management of the
QPC's confinement potential may enable the high g* desirable for spintronic
applications without resorting to narrow-gap materials such as InAs or InSb.
The 0.7 anomaly and zero-bias peak are also highly sensitive to confining
potential, explaining the conflicting density dependencies of the 0.7 anomaly
in the literature.Comment: 23 pages, 7 figure