Spin polarization in the Hubbard model with Rashba spin-orbit coupling on a ladder

The competition between on-site Coulomb repulsion and Rashba spin-orbit coupling (RSOC) is studied on two-leg ladders by numerical techniques. By studying persistent currents in closed rings by exact diagonalization, it is found that the contribution to the current due to the RSOC V_{SO}, for a fixed value of the Hubbard repulsion U reaches a maximum at intermediate values of V_{SO}. By increasing the repulsive Hubbard coupling U, this spin-flipping current is suppressed and eventually it becomes opposite to the spin-conserving current. The main result is that the spin accumulation defined as the relative spin polarization between the two legs of the ladder is enhanced by U. Similar results for this Hubbard-Rashba model are observed for a completely different setup in which two halves of the ladders are connected to a voltage bias and the ensuing time-dependent regime is studied by the density matrix-renormalization group technique. It is also interesting a combined effect between V_{SO} and U leading to a strong enhancement of antiferromagnetic order which in turn may explain the observed behavior of the spin-flipping current. The implications of this enhancement of the spin-Hall effect with electron correlations for spintronic devices is discussed.Comment: 7 pages, 8 figure

El problema con las potestades de fiscalización ambiental: ¿dèjá vu?

The fire at the Santa Marta landfill revealed a serious problem about the control of environmental permits in facilities. An interpretation of the environmental re-form which holds that the SMA has sole and exclusive supervisory powers over the envi-ronmental integrated permit –environmental approval resolution–, and that preceding an administrative secondment agreement of supervisory actions, and a subsequent supervisory subprogram, it can be overseen by other administrative agencies; produced an inhibition in the on-field control activities of the latter, and in a neglect of the reviewing of follow-up reports sent in by the operator, resulting in a virtual inevitability of the fire. This paper propose an interpretative solution for this problem.El incendio en el relleno sanitario Santa Marta reveló un serio problema respecto de la fiscalización de permisos ambientales en instalaciones. La interpretación de la reforma ambiental que sostiene que la SMA tiene competencia exclusiva y excluyente de la fiscalización del permiso ambiental integrado -resolución de calificación ambiental-, y que previo a un convenio administrativo de encomendación de acciones de fiscalización y su subsecuente subprograma de fiscalización, pueden fiscalizarlas otros servicios públicos; resultó en la inhibición de la actividad fiscalizadora en terreno de estos últimos, y en el abandono de la revisión de los informes de seguimiento enviados por el operador, resultan-do en la virtual inevitabilidad del incendio. Este trabajo propone una solución interpretativa para este problema

The Role of the Judiciary Fighting “The Right to Be Ignorant” in Environmental Administrative Proceedings in Chile

Chilean laws provide adequate public access to environmental information, but also provide exemptions from disclosure. The impairment and the trade secret exemptions can hinder transparency and public input when required in environmental-related adjudication and rule-making proceedings. Even they may be misused to avoid blame, therefore information not covered by these exemptions, yet sensitive for any reason, may be unduly withheld. While there is a public transparency watchdog, the Council for Transparency, issues regarding public access to information in environmental-related adjudication and rule-making proceedings can be reviewed by the Environmental Courts. This Article outlines some aspects of the Chilean general transparency and administrative procedures, and the environmental transparency rules, as related to public input in environmental adjudication and rule-making proceedings. Then it shows how the Judiciary has fought “the right to be ignorant”

Absolute Maximal Entanglement and Quantum Secret Sharing

We study the existence of absolutely maximally entangled (AME) states in quantum mechanics and its applications to quantum information. AME states are characterized by being maximally entangled for all bipartitions of the system and exhibit genuine multipartite entanglement. With such states, we present a novel parallel teleportation protocol which teleports multiple quantum states between groups of senders and receivers. The notable features of this protocol are that (i) the partition into senders and receivers can be chosen after the state has been distributed, and (ii) one group has to perform joint quantum operations while the parties of the other group only have to act locally on their system. We also prove the equivalence between pure state quantum secret sharing schemes and AME states with an even number of parties. This equivalence implies the existence of AME states for an arbitrary number of parties based on known results about the existence of quantum secret sharing schemes.Comment: 5 pages, 2 figure

On the soliton width in the incommensurate phase of spin-Peierls systems

We study using bosonization techniques the effects of frustration due to competing interactions and of the interchain elastic couplings on the soliton width and soliton structure in spin-Peierls systems. We compare the predictions of this study with numerical results obtained by exact diagonalization of finite chains. We conclude that frustration produces in general a reduction of the soliton width while the interchain elastic coupling increases it. We discuss these results in connection with recent measurements of the soliton width in the incommensurate phase of CuGeO_3.Comment: 4 pages, latex, 2 figures embedded in the tex

Influence of the anion potential on the charge ordering in quasi-one dimensional charge transfer salts

We examine the various instabilities of quarter-filled strongly correlated electronic chains in the presence of a coupling to the underlying lattice. To mimic the physics of the (TMTTF)$_2$X Bechgaard-Fabre salts we also include electrostatic effects of intercalated anions. We show that small displacements of the anion can stabilize new mixed Charged Density Wave-Bond Order Wave phases in which central symmetry centers are suppressed. This finding is discussed in the context of recent experiments. We suggest that the recently observed charge ordering is due to a cooperative effect between the Coulomb interaction and the coupling of the electronic stacks to the anions. On the other hand, the Spin-Peierls instability at lower temperature requires a Peierls-like lattice coupling.Comment: Latex, 4 pages, 4 postscript figure

Optical conductivity of the Hubbard model at finite temperature

The optical conductivity, $\sigma(\omega)$, of the two dimensional one-band Hubbard model is calculated at finite temperature using exact diagonalization techniques on finite clusters. The in-plane d.c. resistivity, $\rho_{ab}$, is also evaluated. We find that at large U/t and temperature T, $\rho_{ab}$ is approximately linear with temperature, in reasonable agreement with experiments on high-T$_c$ superconductors. Moreover, we note that $\sigma(\omega)$ displays charge excitations, a mid-infrared (MIR) band and a Drude peak, also as observed experimentally. The combination of the Drude peak and the MIR oscillator strengths leads to a conductivity that decays slower than $1/\omega^2$ at energies smaller than the insulator gap near half-filling.Comment: 12 pages, 3 figures appended, Revtex version 2.0, preprin

Effects of domain walls on hole motion in the two-dimensional t-J model at finite temperature

The t-J model on the square lattice, close to the t-J_z limit, is studied by quantum Monte Carlo techniques at finite temperature and in the underdoped regime. A variant of the Hoshen-Koppelman algorithm was implemented to identify the antiferromagnetic domains on each Trotter slice. The results show that the model presents at high enough temperature finite antiferromagnetic (AF) domains which collapse at lower temperatures into a single ordered AF state. While there are domains, holes would tend to preferentially move along the domain walls. In this case, there are indications of hole pairing starting at a relatively high temperature. At lower temperatures, when the whole system becomes essentially fully AF ordered, at least in finite clusters, holes would likely tend to move within phase separated regions. The crossover between both states moves down in temperature as doping increases and/or as the off-diagonal exchange increases. The possibility of hole motion along AF domain walls at zero temperature in the fully isotropic t-J is discussed.Comment: final version, to appear in Physical Review

Climatic and land use changes on the NW of Iberian Peninsula recorded in a 1500-year record from Lake Sanabria

L'estudi de diversos descriptors paleoambientals com ara el pol·len, les diatomees i la sedimentologia, realitzat en sediments procedents del llac de Sanabria (NO de la península Ibèrica), ha aportat informació sobre les oscil·lacions climàtiques atribuïdes als períodes càlids tardoromà i medieval, així com a la petita edat del gel. Entre els anys 440 i 950 dC, el clima es caracteritzà per temperatures suaus i un règim de precipitacions mediterrani, malgrat l'existència de pulsacions més fredes vers els anys 530 i 700 dC. Les evidències pol·líniques dels usos del sòl indiquen l'extensió d'activitats ramaderes i agrícoles. Aquesta fase correspon al final del període càlid romà i al període càlid medieval. El canvi de condicions climàtiques es produeix entre els anys 950 i 1100 dC, moment en què els valors mínims de matèria orgànica, pol·len arbori, concentració de diatomees, nitrogen total (TN) i mida del gra indiquen temperatures més baixes i un règim de precipitacions més regular. Aquest període correspon a la petita edat del gel, que finalitzà vers l'any 1590 dC. Posteriorment, la productivitat del llac tendeix a recuperar els valors previs, malgrat que es produeixen episodis freds i curts. Els valors de carbó orgànic total, TN i diatomees covarien amb els índexs de temperatura del NO de la península Ibèrica i posarien de manifest que, amb anterioritat a l'era industrial, el sistema lacustre de Sanabria estava controlat principalment per les condicions climàtiques. Des de l'any 1920 dC, la productivitat del llac està influenciada per l'activitat humana.This multi-proxy paleoenvironmental study from Lake Sanabria (NW Iberian Peninsula), based on pollen, diatom, and sedimentology, provides evidences of climatic oscillations attributed to the Late Roman and Medieval Warm Periods as well as the Little Ice Age (LIA). From 440 to 950 AD, the climate was characterized by mild temperatures and a Mediterranean rainfall regime, although climatic cold periods were recorded at ca. 530 and 700. Evidence from pollen indicators of land-use suggests that grazing and farming were widespread activities. This period corresponds to the end of the Roman Warm Period and the Medieval Warm Period. The onset of new climate conditions occurred between 950 and 1100 AD, as minimum values of organic matter, arboreal pollen, diatom concentration, total nitrogen (TN), and grain size indicate low temperatures and a more regular rainfall regime. This period corresponds to the LIA and ended at 1590 AD, when lake productivity tended to recover to previous values in spite of the occurrence of cool events. Total organic carbon, TN, and diatom content covary with the temperature index for the NW Iberia, suggesting that Lake Sanabria was mainly controlled by climate before the industrial period. Since 1920 AD, lake productivity has been mainly influenced by human activity

Rapid Suppression of the Spin Gap in Zn-doped CuGeO_3 and SrCu_2O_3

The influence of non-magnetic impurities on the spectrum and dynamical spin structure factor of a model for CuGeO$_3$ is studied. A simple extension to Zn-doped ${\rm Sr Cu_2 O_3}$ is also discussed. Using Exact Diagonalization techniques and intuitive arguments we show that Zn-doping introduces states in the Spin-Peierls gap of CuGeO$_3$. This effect can beunderstood easily in the large dimerization limit where doping by Zn creates ``loose'' S=1/2 spins, which interact with each other through very weak effective antiferromagnetic couplings. When the dimerization is small, a similar effect is observed but now with the free S=1/2 spins being the resulting S=1/2 ground state of severed chains with an odd number of sites. Experimental consequences of these results are discussed. It is interesting to observe that the spin correlations along the chains are enhanced by Zn-doping according to the numerical data presented here. As recent numerical calculations have shown, similar arguments apply to ladders with non-magnetic impurities simply replacing the tendency to dimerization in CuGeO$_3$ by the tendency to form spin-singlets along the rungs in SrCu$_2$O$_3$.Comment: 7 pages, 8 postscript figures, revtex, addition of figure 8 and a section with experimental predictions, submmited to Phys. Rev. B in May 199