1,247 research outputs found
Interaction-induced charge and spin pumping through a quantum dot at finite bias
We investigate charge and spin transport through an adiabatically driven,
strongly interacting quantum dot weakly coupled to two metallic contacts with
finite bias voltage. Within a kinetic equation approach, we identify
coefficients of response to the time-dependent external driving and relate
these to the concepts of charge and spin emissivities previously discussed
within the time-dependent scattering matrix approach. Expressed in terms of
auxiliary vector fields, the response coefficients allow for a straightforward
analysis of recently predicted interaction-induced pumping under periodic
modulation of the gate and bias voltage [Phys. Rev. Lett. 104, 226803 (2010)].
We perform a detailed study of this effect and the related adiabatic Coulomb
blockade spectroscopy, and, in particular, extend it to spin pumping. Analytic
formulas for the pumped charge and spin in the regimes of small and large
driving amplitude are provided for arbitrary bias. In the absence of a magnetic
field, we obtain a striking, simple relation between the pumped charge at zero
bias and at bias equal to the Coulomb charging energy. At finite magnetic
field, there is a possibility to have interaction-induced pure spin pumping at
this finite bias value, and generally, additional features appear in the pumped
charge. For large-amplitude adiabatic driving, the magnitude of both the pumped
charge and spin at the various resonances saturate at values which are
independent of the specific shape of the pumping cycle. Each of these values
provide an independent, quantitative measurement of the junction asymmetry.Comment: 17 pages, 8 figure
Frequency-dependent attenuation and elasticity in unconsolidated earth materials: effect of damping
We use the Discrete Element Method (DEM) to understand the underlying
attenuation mechanism in granular media, with special applicability to the
measurements of the so-called effective mass developed earlier. We consider
that the particles interact via Hertz-Mindlin elastic contact forces and that
the damping is describable as a force proportional to the velocity difference
of contacting grains. We determine the behavior of the complex-valued normal
mode frequencies using 1) DEM, 2) direct diagonalization of the relevant
matrix, and 3) a numerical search for the zeros of the relevant determinant.
All three methods are in strong agreement with each other. The real and the
imaginary parts of each normal mode frequency characterize the elastic and the
dissipative properties, respectively, of the granular medium. We demonstrate
that, as the interparticle damping, , increases, the normal modes exhibit
nearly circular trajectories in the complex frequency plane and that for a
given value of they all lie on or near a circle of radius centered on
the point in the complex plane, where . We show that each
normal mode becomes critically damped at a value of the damping parameter , where is the (real-valued) frequency when
there is no damping. The strong indication is that these conclusions carry over
to the properties of real granular media whose dissipation is dominated by the
relative motion of contacting grains. For example, compressional or shear waves
in unconsolidated dry sediments can be expected to become overdamped beyond a
critical frequency, depending upon the strength of the intergranular damping
constant.Comment: 28 pages, 7 figure
Antiferromagnetic spin chain behavior and a transition to 3D magnetic order in Cu(D,L-alanine)2: Roles of H-bonds
We study the spin chain behavior, a transition to 3D magnetic order and the
magnitudes of the exchange interactions for the metal-amino acid complex
Cu(D,L-alanine)2.H2O, a model compound to investigate exchange couplings
supported by chemical paths characteristic of biomolecules. Thermal and
magnetic data were obtained as a function of temperature (T) and magnetic field
(B0). The magnetic contribution to the specific heat, measured between 0.48 and
30 K, displays above 1.8 K a 1D spin-chain behavior that can be fitted with an
intrachain antiferromagnetic (AFM) exchange coupling constant 2J0 = (-2.12
0.08) cm, between neighbor coppers at 4.49 {\AA} along chains
connected by non-covalent and H-bonds. We also observe a narrow specific heat
peak at 0.89 K indicating a phase transition to a 3D magnetically ordered
phase. Magnetization curves at fixed T = 2, 4 and 7 K with B0 between 0 and 9
T, and at T between 2 and 300 K with several fixed values of B0 were globally
fitted by an intrachain AFM exchange coupling constant 2J0 = (-2.27 0.02)
cm and g = 2.091 0.005. Interchain interactions J1 between coppers
in neighbor chains connected through long chemical paths with total length of
9.51 {\AA} are estimated within the range 0.1 < |2J1| < 0.4 cm, covering
the predictions of various approximations. We analyze the magnitudes of 2J0 and
2J1 in terms of the structure of the corresponding chemical paths. The main
contribution in supporting the intrachain interaction is assigned to H-bonds
while the interchain interactions are supported by paths containing H-bonds and
carboxylate bridges, with the role of the H-bonds being predominant. We compare
the obtained intrachain coupling with studies of compounds showing similar
behavior and discuss the validity of the approximations allowing to calculate
the interchain interactions.Comment: 10 pages, 4 figure
Laser-induced effects on the electronic features of graphene nanoribbons
We study the interplay between lateral confinement and photon-induced
processes on the electronic properties of illuminated graphene nanoribbons. We
find that by tuning the device setup (edges geometries, ribbon width and
polarization direction), a laser with frequency {\Omega} may either not affect
the electronic structure, or induce bandgaps or depletions at \hbar {\Omega}/2,
and/or at other energies not commensurate with half the photon energy. Similar
features are also observed in the dc conductance, suggesting the use of the
polarization direction to switch on and off the graphene device. Our results
could guide the design of novel types of optoelectronic nano-devices.Comment: 4 pages, 3 figure
Time-delay systems that defy intuition: nonrobust forward completeness and related (non)properties
An example of a time-invariant time-delay system that is uniformly globally
attractive and Lyapunov stable, hence forward complete, but whose reachability
sets from bounded initial conditions are not bounded over compact time
intervals is provided. This gives a negative answer to two current conjectures
by showing that (i) forward completeness is not equivalent to robust forward
completeness and (ii) global asymptotic stability is not equivalent to uniform
global asymptotic stability. In addition, a novel characterization of robust
forward completeness for usually encountered classes of time-delay systems is
provided. This characterization relates robust forward completeness of the
time-delay system with the forward completeness of an associated nondelayed
finite-dimensional system.Comment: Submitted to Automatica. Print ISSN: 0005-1098, Online ISSN:
1873-283
Non-perturbative laser effects on the electrical properties of graphene nanoribbons
The use of Floquet theory combined with a realistic description of the
electronic structure of illuminated graphene and graphene nanoribbons is
developed to assess the emergence of non-adiabatic and non-perturbative effects
on the electronic properties. Here, we introduce an efficient computational
scheme and illustrate its use by applying it to graphene nanoribbons in the
presence of both linear and circular polarization. The interplay between
confinement due to the finite sample size and laser-induced transitions is
shown to lead to sharp features on the average conductance and density of
states. Particular emphasis is given to the emergence of the bulk limit
response.Comment: 14 pages, 8 figures, to appear in J. Phys.: Condens. Matter, special
issue on "Ultrafast and nonlinear optics in carbon nanomaterials
LIVED EXPERIENCES OF NON-TLE TEACHERS TEACHING TLE SUBJECTS: A PHENOMENOLOGICAL INQUIRY
The research study focused on the lived experiences of non-TLE teachers teaching TLE subject, how they cope with their present situation, and the insights they can share with other teachers who will experience the same experiences they have. This study used the qualitative phenomenological method of research to explore in depth the experiences, coping strategies and personal insights which can be shared by the participants. The research study purposely identified 12 teachers for the focus group discussion who are all teaching in the 5 public national high schools in the Municipality of Kiblawan, Davao del Sur. The results show that the non-TLE teachers teaching TLE subjects struggled so much in their present teaching career considering that what they are teaching is not their field of specialization, they lack the knowledge and expertise to teach TLE subjects coupled with the lack of insufficient resources and TLE equipment to be used for the subjects being handled. As their coping strategies, the TLE teachers explored various teaching strategies and methods by using technology as a tool in their class discussion, demonstration of required skills and equipment needed for the subjects. They also asked for assistance from their colleagues and some teachers who are experts in TLE subjects and most importantly, they study more every day and be ready for the class. Being resourceful, continuing learning, manifesting courage and application of pedagogical alignment are among the insights shared by the non-TLE teachers teaching TLE subjects. Article visualizations
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