Top and side gated epitaxial graphene field effect transistors

Three types of first generation epitaxial graphene field effect transistors (FET) are presented and their relative merits are discussed. Graphene is epitaxially grown on both the carbon and silicon faces of hexagonal silicon carbide and patterned with electron beam lithography. The channels have a Hall bar geometry to facilitate magnetoresistance measurements. FETs patterned on the Si-face exhibit off-to-on channel resistance ratios that exceed 30. C-face FETs have lower off-to-on resistance ratios, but their mobilities (up to 5000 cm2/Vs) are much larger than that for Si-face transistors. Initial investigations into all-graphene side gate FET structures are promising

Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone's lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological/trivial nature of a possible gap opening. Here we report on a combined optical and transport study of ZrTe5, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the a-c plane, while remaining relatively flat and parabolic in the third direction (along the b axis). Therefore, the electronic states in ZrTe5 cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment

Entanglement, quantum phase transition and scaling in XXZ chain

Motivated by recent development in quantum entanglement, we study relations among concurrence $C$, SU$_q$(2) algebra, quantum phase transition and correlation length at the zero temperature for the XXZ chain. We find that at the SU(2) point, the ground state possess the maximum concurrence. When the anisotropic parameter $\Delta$ is deformed, however, its value decreases. Its dependence on $\Delta$ scales as $C=C_0-C_1(\Delta-1)^2$ in the XY metallic phase and near the critical point (i.e. $1<\Delta<1.3$) of the Ising-like insulating phase. We also study the dependence of $C$ on the correlation length $\xi$, and show that it satisfies $C=C_0-1/2\xi$ near the critical point. For different size of the system, we show that there exists a universal scaling function of $C$ with respect to the correlation length $\xi$.Comment: 4 pages, 3 figures. to appear in Phys. Rev.

Impacto do monocultivo da soja transg?nica na agricultura camponesa do assentamento Rondinha-J?ia/RS

El presente trabajo se trata de un estudio de caso, que tiene por objetivo analizar los impactos que la soja transg?nica trajo para la agricultura campesina del Asentamiento Rondinha, ubicado en el municipio de J?ia en el estado de Rio Grande do Sul. La producci?n de la soja transg?nica fue implantada en el asentamiento en las d?cadas de 1990 y 2000 y desde entonces los agricultores campesinos vienen viviendo los impactos s?cio - ambientales y econ?micos de esta nueva forma de producci?n. A partir de las dificultades encontradas por los campesinos con la producci?n de la soja surge la importancia de un an?lisis m?s profundas sobre los impactos de la producci?n de los transg?nicos y los cambios que este modelo de producci?n trae para familias campesinas del Asentamiento Rondinha. Para ello se utiliz? una metodolog?a de investigaci?n cuali-cuantitativa, un estudio de caso con un itinerario de campo, entrevistando las familias asentadas que est?n directamente ligadas a la producci?n transg?nica, en la b?squeda por entender los motivos que llevaron a los agricultores a producir tal monocultivo y los impactos que la misma trae para los campesinos. Se verific? con el presente trabajo que la producci?n transg?nica afect? de forma negativa a las familias, descaracterizando la agricultura campesina y perjudicando el medio ambiente, debido a la gran cantidad de agrot?xicos utilizados para la producci?n. La monocultura interfiri? en la soberan?a alimentaria de la familia y en la autonom?a del agricultor, en las decisiones dentro de su propiedad, adem?s de que es un sistema de producci?n poco rentable para las peque?as propriedades, debido a los altos costos de producci?n por hect?rea. Se concluye, por lo tanto, con ese trabajo que los beneficios de la producci?n transg?nica para los agricultores son muy peque?os si se comparan los impactos que la misma trae para las familias asentadas, demostrando la importancia de preservar la agricultura campesina que es una de las formas de los agricultores enfrentarse el avance del capital en el campo brasile?o. La organizaci?n de las familias en las cooperativas y en los grupos de mujeres, as? como las actividades comunitarias nos permite entender la capacidad organizacional y econ?mica de los campesinos, que con sus diferentes formas de resistencia permean los tiempos enfrentando el capitalismo, actualmente conocido como el agroneg?cio.O presente trabalho trata se de um estudo de caso, que tem por objetivo analisar os impactos que a soja transg?nica trouxe para a agricultura camponesa do Assentamento Rondinha, localizado no munic?pio de J?ia no estado do Rio Grande do Sul. A produ??o da soja transg?nica foi implantada no assentamento nas d?cadas de 1990 e 2000 e desde ent?o os agricultores camponeses vem vivenciando os impactos s?cio ambientais e econ?micos dessa nova forma de produ??o. A partir das dificuldades encontradas pelos camponeses com a produ??o da soja surge a import?ncia de uma an?lise mais profunda sobre os impactos da produ??o dos transg?nicos e as mudan?as que este modelo de produ??o traz para fam?lias camponesas do Assentamento Rondinha. Para tanto utilizou-se uma metodologia de pesquisa quali-quantitativa, um estudo de caso com um roteiro de campo, entrevistando fam?lias assentadas que est?o diretamente ligadas a produ??o transg?nica, na busca por entender os motivos que levaram os agricultores a produzirem tal monocultura e os impactos que a mesma trouxe para os camponeses. Verificou-se com o presente trabalho que a produ??o transg?nica afetou de forma negativa as fam?lias, descaracterizando a agricultura camponesa e prejudicando o meio ambiente, devido ? grande quantidade de agrot?xicos usado para a produ??o. A monocultura interferiu na soberania alimentar da fam?lia e na autonomia do agricultor, nas decis?es dentro sua propriedade, al?m de que ? um sistema de produ??o pouco rent?vel para as pequenas propriedades devido aos altos custos de produ??o por hectare. Conclui se, portanto, que os benef?cios da produ??o transg?nica para os agricultores s?o muito pequenos se comparados os impactos que a mesma traz para as fam?lias assentadas, demonstrando a import?ncia de se preservar a agricultura camponesa que ? uma das formas dos agricultores enfrentar o avan?o do capital no campo brasileiro. A organiza??o das fam?lias nas cooperativas e nos grupos de mulheres, assim como as atividades comunit?rias nos permite entender a capacidade organizacional e econ?mica dos camponeses, que com as suas diferentes formas de resist?ncia permeiam os tempos enfrentando o capital atualmente conhecido como o agroneg?cio

Higgs algebraic symmetry of screened system in a spherical geometry

The orbits and the dynamical symmetries for the screened Coulomb potentials and isotropic harmonic oscillators have been studied by Wu and Zeng [Z. B. Wu and J. Y. Zeng, Phys. Rev. A 62,032509 (2000)]. We find the similar properties in the responding systems in a spherical space, whose dynamical symmetries are described by Higgs Algebra. There exists a conserved aphelion and perihelion vector, which, together with angular momentum, constitute the generators of the geometrical symmetry group at the aphelia and perihelia points $(\dot{r}=0)$.Comment: 8 pages, 1 fi

Comparison between two mobile absolute gravimeters: optical versus atomic interferometers

We report a comparison between two absolute gravimeters: the LNE-SYRTE cold atoms gravimeter and FG5#220 of Leibniz Universit\"at of Hannover. They rely on different principles of operation: atomic and optical interferometry. Both are movable which enabled them to participated to the last International Comparison of Absolute Gravimeters (ICAG'09) at BIPM. Immediately after, their bilateral comparison took place in the LNE watt balance laboratory and showed an agreement of 4.3 +/- 6.4 {\mu}Gal

How to Identify and Separate Bright Galaxy Clusters from the Low-frequency Radio Sky?

In this work we simulate the $50-200$ MHz radio sky that is constrained in the field of view ($5^{\circ}$ radius) of the 21 Centimeter Array (21CMA), by carrying out Monte-Carlo simulations to model redshifted cosmological reionization signals and strong contaminating foregrounds, including emissions from our Galaxy, galaxy clusters, and extragalactic point sources. As an improvement of previous works, we consider in detail not only random variations of morphological and spectroscopic parameters within the ranges allowed by multi-band observations, but also evolution of radio halos in galaxy clusters, assuming that relativistic electrons are re-accelerated in the ICM in merger events and lose energy via both synchrotron emission and inverse Compton scattering with CMB photons. By introducing a new approach designed on the basis of independent component analysis (ICA) and wavelet detection algorithm, we prove that, with a cumulative observation of one month with the 21CMA array, about $80\%$ of galaxy clusters with central brightness temperatures of $> 10~{\rm K}$ at 65 MHz can be safely identified and separated from the overwhelmingly bright foreground. We find that the morphological and spectroscopic distortions are extremely small as compared to the input simulated clusters, and the reduced $\chi^2$ of brightness temperature profiles and spectra are controlled to be $\lesssim 0.5$ and $\lesssim 1.3$, respectively. These results robustly indicate that in the near future a sample of dozens of bright galaxy clusters will be disentangled from the foreground in 21CMA observations, the study of which will greatly improve our knowledge about cluster merger rates, electron acceleration mechanisms in cluster radio halos, and magnetic field in the ICM.Comment: 35 pages, 10 figures, Accepted for publication in The Astrophysical Journa

Analytical results for entanglement in the five-qubit anisotropic Heisenberg model

We solve the eigenvalue problem of the five-qubit anisotropic Heisenberg model, without use of Bethe's Ansatz, and give analytical results for entanglement and mixedness of two nearest-neighbor qubits. The entanglement takes its maximum at Delta= (Delta>1) for the case of zero (finite) temperature with Delta being the anisotropic parameter. In contrast, the mixedness takes its minimum at Delta=1 (Delta>1) for the case of zero (finite) temperature.Comment: Four pages, three figure

Hot Jupiters and Hot Spots: The Short- and Long-term Chromospheric Activity on Stars with Giant Planets

We monitored the chromospheric activity in the Ca II H & K lines of 13 solar-type stars (including the Sun); 8 of them over three years at the CFHT and 5 in a single run at the VLT. Ten of the 13 targets have close planetary companions. All of the stars observed at the CFHT show long-term (months to years) changes in H & K intensity levels. Four stars display short-term (days) cyclical activity. For two, HD 73256 and kappa^1 Ceti, the activity is likely associated with an active region rotating with the star, however, the flaring in excess of the rotational modulation may be associated with a hot jupiter. A planetary companion remains a possibility for kappa^1 Ceti. For the other two, HD 179949 and upsilon And, the cyclic variation is synchronized to the hot jupiter's orbit. For both stars this synchronicity with the orbit is clearly seen in two out of three epochs. The effect is only marginal in the third epoch at which the seasonal level of chromospheric activity had changed for both stars. Short-term chromospheric activity appears weakly dependent on the mean K-line reversal intensities for the sample of 13 stars. Also, a suggestive correlation exists between this activity and the M_p sin(i) of the star's hot jupiter. Because of their small separation (<= 0.1 AU), many of the hot jupiters lie within the Alfv\'en radius of their host stars which allows a direct magnetic interaction with the stellar surface. We discuss the conditions under which a planet's magnetic field might induce activity on the stellar surface and why no such effect was seen for the prime candidate, tau Boo. This work opens up the possibility of characterizing planet-star interactions, with implications for extrasolar planet magnetic fields and the energy contribution to stellar atmospheres.Comment: Accepted to the Astrophysical Journal; 39 pages including 17 figure

Hot-Jupiters and hot-Neptunes: a common origin?

We compare evolutionary models for close-in exoplanets coupling irradiation and evaporation due respectively to the thermal and high energy flux of the parent star with observations of recently discovered new transiting planets. The models provide an overall good agreement with observations, although at the very limit of the quoted error bars of OGLE-TR-10, depending on its age. Using the same general theory, we show that the three recently detected hot-Neptune planets (GJ436, $\rho$ Cancri, $\mu$ Ara) may originate from more massive gas giants which have undergone significant evaporation. We thus suggest that hot-Neptunes and hot-Jupiters may share the same origin and evolution history. Our scenario provides testable predictions in terms of the mass-radius relationships of these hot-Neptunes.Comment: 5 pages, 2 figures, accepted in A&A Lette