1,508 research outputs found
Far-infrared absorption and the metal-to-insulator transition in hole-doped cuprates
By studying the optical conductivity of BSLCO and YCBCO, we show that the
metal-to-insulator transition (MIT) in these hole-doped cuprates is driven by
the opening of a small gap at low T in the far infrared. Its width is
consistent with the observations of Angle-Resolved Photoemission Spectroscopy
in other cuprates, along the nodal line of the k-space. The gap forms as the
Drude term turns into a far-infrared absorption, whose peak frequency can be
approximately predicted on the basis of a Mott-like transition. Another band in
the mid infrared softens with doping but is less sensitive to the MIT.Comment: To be published on Physical Review Letter
Low loss Ge-on-Si waveguides operating in the 8–14 µm atmospheric transmission window
Germanium-on-silicon waveguides were modeled, fabricated and characterized at wavelengths ranging from 7.5 to 11 µm. Measured waveguide losses are below 5 dB/cm for both TE and TM polarization and reach values of ∼ 1 dB/cm for ≥ 10 µm wavelengths for the TE polarization. This work demonstrates experimentally for the first time that Ge-on-Si is a viable waveguide platform for sensing in the molecular fingerprint spectral region. Detailed modeling and analysis is presented to identify the various loss contributions, showing that with practical techniques losses below 1 dB/cm could be achieved across the full measurement range
Modelling mental rotation in cognitive robots
Mental rotation concerns the cognitive processes that allow an agent mentally to rotate the image of an object in order to solve a given task, for example to say if two objects with different orientations are the same or different. Here we present a system-level bio-constrained model, developed within a neurorobotics framework, that provides an embodied account of mental rotation processes relying on neural mechanisms involving motor affordance encoding, motor simulation and the anticipation of the sensory consequences of actions (both visual and proprioceptive). This model and methodology are in agreement with the most recent theoretical and empirical research on mental rotation. The model was validated through experiments with a simulated humanoid robot (iCub) engaged in solving a classical mental rotation test. The results of the test show that the robot is able to solve the task and, in agreement with data from psychology experiments, exhibits response times linearly dependent on the angular disparity between the objects. This model represents a novel detailed operational account of the embodied brain mechanisms that may underlie mental rotation. © The Author(s) 2013
Electrodynamics near the Metal-to-Insulator Transition in V3O5
The electrodynamics near the metal-to-insulator transitions (MIT) induced, in
V3O5 single crystals, by both temperature (T) and pressure (P) has been studied
by infrared spectroscopy. The T- and P-dependence of the optical conductivity
may be explained within a polaronic scenario. The insulating phase at ambient T
and P corresponds to strongly localized small polarons. Meanwhile the T-induced
metallic phase at ambient pressure is related to a liquid of polarons showing
incoherent dc transport, in the P-induced metallic phase at room T strongly
localized polarons coexist with partially delocalized ones. The electronic
spectral weight is almost recovered, in both the T and P induced metallization
processes, on an energy scale of 1 eV, thus supporting the key-role of
electron-lattice interaction in the V3O5 metal-to-insulator transition.Comment: 7 pages, 5 figure
Optical Properties of (SrMnO3)n/(LaMnO3)2n superlattices: an insulator-to-metal transition observed in the absence of disorder
We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices
(SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator
to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic
mid-infrared band. At n = 5 that softening is incomplete, while at the
largest-period n=8 compound the MIR band is independent of T and the SL remains
insulating. One can thus first observe the IMT in a manganite system in the
absence of the disorder due to chemical doping. Unsuccessful reconstruction of
the SL optical properties from those of the original bulk materials suggests
that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic
state.Comment: Published Online in Nano Letters, November 8, 2010;
http://pubs.acs.org/doi/abs/10.1021/nl1022628; 5 pages, 3 figure
Optical properties of V2O3 in its whole phase diagram
Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard
physics. In this paper we present an extended optical study of its whole
temperature/doping phase diagram as obtained by doping the pure material with
M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable
metallic and insulating phases, although macroscopically equivalent, show very
different low-energy electrodynamics. The Cr and Ti doping drastically change
both the antiferromagnetic gap and the paramagnetic metallic properties. A
slight chromium content induces a mesoscopic electronic phase separation, while
the pure compound is characterized by short-lived quasiparticles at high
temperature. This study thus provides a new comprehensive scenario of the
Mott-Hubbard physics in the prototype compound V2O3
How affordances associated with a distractor object affect compatibility effects: a study with the computational model TRoPICALS.
Seeing an object activates both visual and action codes in the brain. Crucial evidence supporting this view is the observation of object to response compatibility effects: perception of an object can facilitate or interfere with the execution of an action (e.g., grasping) even when the viewer has no intention of interacting with the object. TRoPICALS is a computational model that proposes some general principles about the brain mechanisms underlying compatibility effects, in particular the idea that top-down bias from prefrontal cortex, and whether it conflicts or not with the actions afforded by an object, plays a key role in such phenomena. Experiments on compatibility effects using a target and a distractor object show the usual positive compatibility effect of the target, but also an interesting negative compatibility effect of the distractor: responding with a grip compatible with the distractor size produces slower reaction times than the incompatible case. Here, we present an enhanced version of TRoPICALS that reproduces and explains these new results. This explanation is based on the idea that the prefrontal cortex plays a double role in its top-down guidance of action selection producing: (a) a positive bias in favour of the action requested by the experimental task; (b) a negative bias directed to inhibiting the action evoked by the distractor. The model also provides testable predictions on the possible consequences of damage to volitional circuits such as in Parkinsonian patients
Evidence of a pressure-induced metallization process in monoclinic VO
Raman and combined trasmission and reflectivity mid infrared measurements
have been carried out on monoclinic VO at room temperature over the 0-19
GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence
obtained for both lattice dynamics and optical gap shows a remarkable stability
of the system up to P*10 GPa. Evidence of subtle modifications of V ion
arrangements within the monoclinic lattice together with the onset of a
metallization process via band gap filling are observed for PP*. Differently
from ambient pressure, where the VO metal phase is found only in
conjunction with the rutile structure above 340 K, a new room temperature
metallic phase coupled to a monoclinic structure appears accessible in the high
pressure regime, thus opening to new important queries on the physics of
VO.Comment: 5 pages, 3 figure
Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe_{1.84}Co_{0.16}As_2 ultrathin film: an analysis of the optical gaps in the Co-doped BaFe_2As_2 pnictide
Here we report an optical investigation in the terahertz region of a 40 nm
ultrathin BaFeCoAs superconducting film with
superconducting transition temperature T = 17.5 K. A detailed analysis of
the combined reflectance and transmittance measurements showed that the optical
properties of the superconducting system can be described in terms of a
two-band, two-gap model. The zero temperature value of the large gap
, which seems to follow a BCS-like behavior, results to be
(0) = 17 cm. For the small gap, for which (0) = 8
cm, the temperature dependence cannot be clearly established. These gap
values and those reported in the literature for the BaFeCoAs
system by using infrared spectroscopy, when put together as a function of
T, show a tendency to cluster along two main curves, providing a unified
perspective of the measured optical gaps. Below a temperature around 20 K, the
gap-sizes as a function of T seem to have a BCS-like linear behavior, but
with different slopes. Above this temperature, both gaps show different
supra-linear behaviors
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