5,993 research outputs found
Spin relaxation in corrugated graphene
In graphene, out-of-plane (flexural) vibrations and static ripples imposed by
the substrate relax the electron spin, intrinsically protected by mirror
symmetry. We calculate the relaxation times in different scenarios, accounting
for all the possible spin-phonon couplings allowed by the hexagonal symmetry of
the lattice. Scattering by flexural phonons imposes the ultimate bound to the
spin lifetimes, in the ballpark of hundreds of nano-seconds at room
temperature. This estimate and the behavior as a function of the carrier
concentration are substantially altered by the presence of tensions or the
pinning with the substrate. Static ripples also influence the spin transport in
the diffusive regime, dominated by motional narrowing. We find that the
D'yakonov-Perel' mechanism saturates when the mean free path is comparable to
the correlation length of the heights profile. In this regime, the
spin-relaxation times are exclusively determined by the geometry of the
corrugations. Simple models for typical corrugations lead to lifetimes of the
order of tens of micro-seconds.Comment: 4 + epsilon pages; 3 figure
Trend analysis in two standard growth models
This paper analyzes the trend processes characterized by two standard growth models using simple econometrics. The first model is the basic neoclassical growth model that postulates a deterministic trend for output. The second model is the Uzawa-Lucas model that postulates a stochastic trend for output. The aim is to understand how the different trend processes for output assumed by these two standard growth models determine the ability of each model to explain the observed trend processes of other macroeconomic variables such as consumption and investment. The results show that the two models reproduce the output trend process. Moreover, the results show that the basic growth model captures properly the consumption trend process, but fails in characterizing the investment trend process. The reverse is true for the Uzawa-Lucas model.Uzawa-Lucas model, basic neoclassical growth model, trend process, cointegration
Cyclical Features of Uzawa-Lucas Endogenous Growth Model
This paper analyzes the cyclical properties of a generalized version of Uzawa-Lucas endogenous growth model. We study the dynamic features of different cyclical components of this model characterized by a variety of decomposition methods. The decomposition methods considered can be classified in two groups. On the one hand, we consider three statistical filters: the Hodrick-Prescott filter, the Baxter-King filter and Gonzalo-Granger decomposition. On the other hand, we use four model-based decomposition methods. The latter decomposition procedures share the property that the cyclical components obtained by these methods preserve the log-linear approximation of the Euler-equation restrictions imposed by the agent’s intertemporal optimization problem. The paper shows that both model dynamics and model performance substantially vary across decomposition methods. A parallel exercise is carried out with a standard real business cycle model. The results should help researchers to better understand the performance of Uzawa-Lucas model in relation to standard business cycle models under alternative definitions of the business cycle.endogenous growth, decomposition methods, cyclical features
Combined photo- and electroreflectance of multijunction solar cells enabled by subcell electric coupling
Electric coupling between subcells of a monolithically grown multijunction
solar cell in short circuit allows their simultaneous and independent
characterization by means of photo- and electroreflectance. The photovoltage
generated by selective absorption of the pump beam in a given subcell during
photoreflectance measurements results in reverse biasing the complementary
subunits at the modulation frequency set on the pump illumination. Such voltage
bias modulation acts then as external perturbation on the complementary
subcells. The spectral separation of the different subcell absorption ranges
permits the probe beam to record in a single spectrum the response of the
complete device as a combination of photo- and electroreflectance, thereby
providing access for diagnosis of subcells on an individual basis. This form of
modulation spectroscopy is demonstrated on a GaInP/GaAs tandem solar cell.Comment: 5 pages, 4 figures. This article has been accepted by Appl. Phys.
Lett. After it is published, it will be found at
https://doi.org/10.1063/1.506260
Elliot-Yafet mechanism in graphene
The differences between spin relaxation in graphene and in other materials
are discussed. For relaxation by scattering processes, the Elliot-Yafet
mechanism, the relation between the spin and the momentum scattering times
acquires a dependence on the carrier density, which is independent of the
scattering mechanism and the relation between mobility and carrier
concentration. This dependence puts severe restrictions on the origin of the
spin relaxation in graphene. The density dependence of the spin relaxation
allows us to distinguish between ordinary impurities and defects which modify
locally the spin-orbit interaction.Comment: 4 pages + \epsilon + S
The importance of the tumor microenvironment to understand tumor origin, evolution, and treatment response
During the second half of the twentieth century, oncology adopted a tumor-centric approach to cancer treatment, focusing primarily on the tumor cell to identify new therapeutic targets. However, since the 2000s, we have seen a gradual shift in this paradigm with numerous studies highlighting the importance of the tumor microenvironment in tumor progression, patient prognosis, and therapy response. Solid tumors are highly complex systems where numerous cell types and microenvironmental factors are intertwined, potentially affecting tumor evolution, treatment response, and patient outcome. Stromal cells such as fibroblasts and immune cells can stimulate or suppress tumor growth and are currently being used as therapeutic targets in numerous studies and clinical trials..
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