Fixed Point Algebras for Easy Quantum Groups

Compact matrix quantum groups act naturally on Cuntz algebras. The first author isolated certain conditions under which the fixed point algebras under this action are Kirchberg algebras. Hence they are completely determined by their $K$-groups. Building on prior work by the second author, we prove that free easy quantum groups satisfy these conditions and we compute the $K$-groups of their fixed point algebras in a general form. We then turn to examples such as the quantum permutation group $S_n^+$, the free orthogonal quantum group $O_n^+$ and the quantum reflection groups $H_n^{s+}$. Our fixed point-algebra construction provides concrete examples of free actions of free orthogonal easy quantum groups, which are related to Hopf-Galois extensions

High frequency thermoelectric response in correlated electronic systems

We derive a general formalism for evaluating the high-frequency limit of the thermoelectric power of strongly correlated materials, which can be straightforwardly implemented in available first principles LDA+DMFT programs. We explore this formalism using model Hamiltonians and we investigate the validity of approximating the static thermoelectric power $S_0$, by its high-temperature limit, $S^*$. We point out that the behaviors of $S^*$ and $S_0$ are qualitatively different for a correlated Fermi liquid near the Mott transition, when the temperature is in the coherent regime. When the temperature is well above the coherent regime, e.g., when the transport is dominated by incoherent excitations, $S^*$ provides a good estimation of $S_0$.Comment: 23 page

Impurity model for non-equilibrium steady states

We propose an out-of-equilibrium impurity model for the dynamical mean-field description of the Hubbard model driven by a finite electric field. The out-of-equilibrium impurity environment is represented by a collection of equilibrium reservoirs at different chemical potentials. We discuss the validity of the impurity model and propose a non-perturbative method, based on a quantum Monte Carlo solver, which provides the steady-state solutions of the impurity and original lattice problems. We discuss the relevance of this approach to other non-equilibrium steady-state contexts.Comment: 5 pages, 4 figure

One-dimensional p-wave superconductor toy-model for Majorana fermions in multiband semiconductor nanowires

Majorana fermions are particles identical to their antiparticles proposed theoretically in 1937 by Ettore Majorana as real solutions of the Dirac equation. Alexei Kitaev suggested that Majorana particles should emerge in condensed matter systems as zero mode excitations in one-dimensional p-wave superconductors, with possible applications in quantum computation due to their non-abelian statistics. The search for Majorana zero modes in condensed matter systems led to one of the first realistic models based in a semiconductor nanowire with high spin-orbit coupling, induced superconducting s-wave pairing and Zeeman splitting. Soon, it was realized that size-quantization effects should generate subbands in these systems that could even allow the emergence of more than one Majorana mode at each edge, resulting in a zero bias peak on the differential conductance with a different shape from the predicted by simplified theoretical models. In this work, we provide a connection between a finite-size nanowire with two occupied subbands and a 2-band Kitaev chain and discuss the advantage of an one-dimensional model to understand the phenomenology of the system, including the presence of a hidden chiral symmetry and its similarity with a spinfull Kitaev chain under a magnetic field.Comment: 4 pages, 2 figures. IEEE Transactions on Applied Superconductivity accepted versio

Properties of high-density matter in neutron stars

This short review aims at giving a brief overview of various states of matter that have been suggested to exist in the ultra-dense centers of neutron stars. Particular emphasis is put on the role of quark deconfinement in neutron stars and on the possible existence of compact stars made of absolutely stable strange quark matter (strange stars). Astrophysical phenomena, which distinguish neutron stars from quark stars, are discussed and the question of whether or not quark deconfinement may occur in neutron stars is investigated. Combined with observed astrophysical data, such studies are invaluable to delineate the complex structure of compressed baryonic matter and to put firm constraints on the largely unknown equation of state of such matter.Fil: Weber, Fridolin. San Diego State University; Estados Unidos. University of California; Estados UnidosFil: Contrera, Gustavo Aníbal Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Área Física Teórica; Argentina. San Diego State University; Estados UnidosFil: Orsaria, Milva Gabriela. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Área Física Teórica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. San Diego State University; Estados UnidosFil: Spinella, William. San Diego State University. Computational Sciences Research Center; Estados UnidosFil: Zubairi, Omair. San Diego State University. Computational Sciences Research Center; Estados Unido

Stellar rotation, binarity, and lithium in the open cluster IC4756

An important aspect in the evolutionary scenario of cool stars is their rotation and the rotationally induced magnetic activity and interior mixing. Stars in open clusters are particularly useful tracers for these aspects because of their known ages. We aim to characterize the open cluster IC4756 and measure stellar rotation periods and surface differential rotation for a sample of its member stars. Thirty-seven cluster stars were observed continuously with the CoRoT satellite for 78 days in 2010. Follow-up high-resolution spectroscopy of the CoRoT targets and deep Str\"omgren $uvby\beta$ and H$\alpha$ photometry of the entire cluster were obtained with our robotic STELLA facility and its echelle spectrograph and wide-field imager, respectively. We determined high-precision photometric periods for 27 of the 37 CoRoT targets and found values between 0.155 and 11.4 days. Twenty of these are rotation periods. Twelve targets are spectroscopic binaries of which 11 were previously unknown; orbits are given for six of them. Six targets were found that show evidence of differential rotation with $\Delta\Omega/\Omega$ in the range 0.04-0.15. Five stars are non-radially pulsating stars with fundamental periods of below 1d, two stars are semi-contact binaries, and one target is a micro-flaring star that also shows rotational modulation. Nine stars in total were not considered members because of much redder color(s) and deviant radial velocities with respect to the cluster mean. H$\alpha$ photometry indicates that the cluster ensemble does not contain magnetically over-active stars. The cluster average metallicity is -0.08$\pm$0.06 (rms) and its logarithmic lithium abundance for 12 G-dwarf stars is 2.39$\pm$0.17 (rms). [...]Comment: A&A, in pres

The Carbon Market and its Regulation in Brazil

At present, the global geopolitical scenario is based on an economy undergoing a post-pandemic recovery, with inflation unleashed in various developed countries and a war conflict in Europe, which entails an overall increase in energy prices, food insecurity and a breakdown of supply chains. Under such circumstances, the outlook is one of enormous pressure on the mitigation and adaptation plans of the United Nations, which seeks to revert climate warming caused by human actions in the post-industrial revolution era. The watchword is decarbonizing the global economy by reducing greenhouse gas (GHG) emissions and changing carbon-intensive production regimes. Indeed, countries, companies, and organizations need to increase energy efficiency, reduce or eliminate the use of fossil fuels and increase rates of carbon sequestration and long-term carbon storage. While there is consensus that decarbonization is imperative for human survival, post-pandemic realities and international conflicts have created energy and food insecurity that obstruct the path to a low-carbon or even zero-carbon economy. In the world, the economic sectors with the greatest emissions are the Fossil Industry, Agriculture and Transportation. Changes in the production cycle in order to include new technologies that can mitigate emissions do not occur fast enough to reap their benefits, especially because of the economic costs involved in these changes. With a focus on the agricultural sector, this chapter will demonstrate how the carbon market is an effective and economically fundamental way to enable the implementation of protocols, public policies and legal strategies—taking into account the constitutional principles and the most progressive and sustainable precedents laid down by two Brazilian Superior Courts (i.e., the Supreme Federal Court and the Superior Court of Justice, which recognize a balanced environment as a third generation fundamental right or as one of a very new dimension—that will reduce emissions from the agricultural sector while minimizing the costs of these changes, ultimately giving shape to a virtuous economic and environmental circle. Agriculture encompasses human activities for the production of food and fiber that are essential to sustain life on Earth. Because of its capacity to simultaneously emit and sequester carbon, it is necessary to choose appropriate legal strategies and agricultural technologies that encourage and provide neutral or negative emissions. However, there is a cost associated with such changes in these complex production cycles. Data made available by the Intergovernmental Panel on Climate Change (IPCC) show that global population growth and changes in per capita consumption after the Second World War, especially for food, feed, fiber, wood and energy, caused an unprecedented increase in water and land use, with a major expansion of agriculture over forested areas and the loss of ecosystems. On the one hand, there is illegal deforestation within the context of land use, technically defined by the IPCC as the unsuitable management practices of Agriculture, Forestry and Other Land Use (AFOLU) causing erosive and predatory degradation and responsible in themselves for approximately 13 percent of total carbon dioxide and 44 percent of methane (CH4) emissions in the period 2007-2016, i.e., an average of roughly 24 percent of total emissions. On the other hand, sustainable agricultural activities make up an important GHG sink, assuming a central role in decarbonizing the economy. And, in this regard, Brazil can contribute more quickly to carbon sequestration through strict measures against deforestation and the adoption of increasingly advanced and sustainable techniques to replace more carbon-intensive farming practices. As we will see below, a correct regulatory framework can help agriculture put into force new production processes adapted to a low carbon reality and, furthermore, it can also assign a compensatory monetary value for new practices that will generate social and environmental benefits due to emissions mitigation. This new mechanism is called the carbon market