1,659 research outputs found
1.55 μm direct bandgap electroluminescence from strained n-Ge quantum wells grown on Si substrates
Electroluminescence from strained n-Ge quantum well light emitting diodes grown on a silicon substrate are demonstrated at room temperature. Electroluminescence characterisation demonstrates two peaks around 1.55 μm and 1.8 μm, which correspond to recombination between the direct and indirect transitions, respectively. The emission wavelength can be tuned by around 4% through changing the current density through the device. The devices have potential applications in the fields of optical interconnects, gas sensing, and healthcare
Swift chiral quantum walks
A continuous-time quantum walk (CTQW) is sedentary if the return probability
in the starting vertex is close to one at all times. Recent results imply that,
when starting from a maximal degree vertex, the CTQW dynamics generated by the
Laplacian and adjacency matrices are typically sedentary. In this paper, we
show that the addition of appropriate complex phases to the edges of the graph,
defining a chiral CTQW, can cure sedentarity and lead to swift chiral quantum
walks of the adjacency type, which bring the returning probability to zero in
the shortest time possible. We also provide a no-go theorem for swift chiral
CTQWs of the Laplacian type. Our results provide one of the first, general
characterization of tasks that can and cannot be achieved with chiral CTQWs.Comment: 20 pages, 2 figure
Alexander quandle lower bounds for link genera
We denote by Q_F the family of the Alexander quandle structures supported by
finite fields. For every k-component oriented link L, every partition P of L
into h:=|P| sublinks, and every labelling z of such a partition by the natural
numbers z_1,...,z_n, the number of X-colorings of any diagram of (L,z) is a
well-defined invariant of (L,P), of the form q^(a_X(L,P,z)+1) for some natural
number a_X(L,P,z). Letting X and z vary in Q_F and among the labellings of P,
we define a derived invariant A_Q(L,P)=sup a_X(L,P,z).
If P_M is such that |P_M|=k, we show that A_Q(L,P_M) is a lower bound for
t(L), where t(L) is the tunnel number of L. If P is a "boundary partition" of L
and g(L,P) denotes the infimum among the sums of the genera of a system of
disjoint Seifert surfaces for the L_j's, then we show that A_Q(L,P) is at most
2g(L,P)+2k-|P|-1. We set A_Q(L):=A_Q(L,P_m), where |P_m|=1. By elaborating on a
suitable version of a result by Inoue, we show that when L=K is a knot then
A_Q(K) is bounded above by A(K), where A(K) is the breadth of the Alexander
polynomial of K. However, for every g we exhibit examples of genus-g knots
having the same Alexander polynomial but different quandle invariants A_Q.
Moreover, in such examples A_Q provides sharp lower bounds for the genera of
the knots. On the other hand, A_Q(L) can give better lower bounds on the genus
than A(L), when L has at least two components.
We show that in order to compute A_Q(L) it is enough to consider only
colorings with respect to the constant labelling z=1. In the case when L=K is a
knot, if either A_Q(K)=A(K) or A_Q(K) provides a sharp lower bound for the knot
genus, or if A_Q(K)=1, then A_Q(K) can be realized by means of the proper
subfamily of quandles X=(F_p,*), where p varies among the odd prime numbers.Comment: 36 pages; 16 figure
Robot Impedance Control and Passivity Analysis with Inner Torque and Velocity Feedback Loops
Impedance control is a well-established technique to control interaction
forces in robotics. However, real implementations of impedance control with an
inner loop may suffer from several limitations. Although common practice in
designing nested control systems is to maximize the bandwidth of the inner loop
to improve tracking performance, it may not be the most suitable approach when
a certain range of impedance parameters has to be rendered. In particular, it
turns out that the viable range of stable stiffness and damping values can be
strongly affected by the bandwidth of the inner control loops (e.g. a torque
loop) as well as by the filtering and sampling frequency. This paper provides
an extensive analysis on how these aspects influence the stability region of
impedance parameters as well as the passivity of the system. This will be
supported by both simulations and experimental data. Moreover, a methodology
for designing joint impedance controllers based on an inner torque loop and a
positive velocity feedback loop will be presented. The goal of the velocity
feedback is to increase (given the constraints to preserve stability) the
bandwidth of the torque loop without the need of a complex controller.Comment: 14 pages in Control Theory and Technology (2016
A constant dark matter halo surface density in galaxies
We confirm and extend the recent finding that the central surface density
r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core
radius and central density, is nearly constant and independent of galaxy
luminosity. Based on the co-added rotation curves of about 1000 spiral
galaxies, mass models of individual dwarf irregular and spiral galaxies of late
and early types with high-quality rotation curves and, galaxy-galaxy weak
lensing signals from a sample of spiral and elliptical galaxies, we find that
log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the
observed kinematics of Local Group dwarf spheroidal galaxies are consistent
with this value. Our results are obtained for galactic systems spanning over 14
magnitudes, belonging to different Hubble Types, and whose mass profiles have
been determined by several independent methods. In the same objects, the
approximate constancy of rho_0*r_0 is in sharp contrast to the systematical
variations, by several orders of magnitude, of galaxy properties, including
rho_0 and central stellar surface density.Comment: Accepted for publication in MNRAS. 9 pages, 4 figure
Monojet searches for momentum-dependent dark matter interactions
We consider minimal dark matter scenarios featuring momentum-dependent couplings of the dark sector to the Standard Model. We derive constraints from existing LHC searches in the monojet channel, estimate the future LHC sensitivity for an integrated luminosity of 300 fb−1, and compare with models exhibiting conventional momentum-independent interactions with the dark sector. In addition to being well motivated by (composite) pseudo-Goldstone dark matter scenarios, momentum-dependent couplings are interesting as they weaken direct detection constraints. For a specific dark matter mass, the LHC turns out to be sensitive to smaller signal cross-sections in the momentum-dependent case, by virtue of the harder jet transverse-momentum distribution
Mid-infrared intersubband absorption from p-Ge quantum wells grown on Si substrates
Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a Si substrate is demonstrated from 6 to 9 μm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red transmission and photoluminescence measurements demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8–13 μm
Mid-Infrared Intersubband Absorption from P-Ge Quantum Wells on Si
Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a Si substrate is demonstrated from 6 to 9 μm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red spectroscopy measurements demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8–13 μm
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