Mean-field theory of the spin-Peierls systems: Application to CuGeO3

A mean-field theory of the spin Peierls systems based on the two dimensional dimerized Heisenberg model is proposed by introducing an alternating bond order parameter. Improvements with respect to previous mean-field results are found in the one-dimensional limit for the ground state and the gap energies. In two dimensions, the analysis of the competition between antiferromagnetic long range order and the spin-Peierls ordering is given as a function of the coupling constants. We show that the lowest energy gap to be observed does not have a singlet-triplet character in agreement with the low temperature thermodynamic properties of CuGeO3.Comment: 3 Revtex pages. Submitted to Rapid Comm. Figures available upon reques

Spectral Function in Mott Insulating Surfaces

We show theoretically the fingerprints of short-range spiral magnetic correlations in the photoemission spectra of the Mott insulating ground states realized in the triangular silicon surfaces K/Si(111)-B and SiC(0001). The calculated spectra present low energy features of magnetic origin with a reduced dispersion ~10-40 meV compared with the center-of-mass spectra bandwidth ~0.2-0:3 eV. Remarkably, we find that the quasiparticle signal survives only around the magnetic Goldstone modes. Our findings would position these silicon surfaces as new candidates to investigate non-conventional quasiparticle excitations.Comment: 5 pages, 4 figures. To be published in Journal of Physics: Condensed Matte

Tuning the spin Hamiltonian of NENP by external pressure: a neutron scattering study

We report an inelastic neutron scattering study of antiferromagnetic spin dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations in NENP are dominated by a long-lived triplet mode with a gap which is split by orthorhombic crystalline anisotropy into a lower doublet centered at $\Delta_\perp\approx$ 1.2meV and a singlet at $\Delta_\parallel\approx$ 2.5meV. With pressure we observe appreciable shifts in these levels, which move to $\Delta_\perp{(2.5GPa)}\approx$ 1.45 meV and $\Delta_\parallel(2.5GPa)\approx$ 2.2meV. The dispersion of these modes in the crystalline c-direction perpendicular to the chain was measured here for the first time, and can be accounted for by an interchain exchange J'_c approximately 3e-4*J which changes only slightly with pressure. Since the average gap value $\Delta_H\approx$ 1.64 meV remains almost unchanged with P, we conclude that in NENP the application of external pressure does not affect the intrachain coupling J appreciably, but does produce a significant decrease of the single-ion anisotropy constant from D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa.Comment: LaTeX file nenp_p.tex, 10 pages, 1 table, 5 figures. Submitted to Phys. Rev.

Atmospheric pressure gas chromatography-time-of-flight-mass spectrometry (APGC-ToF-MS) for the determination of regulated and emerging contaminants in aqueous samples after stir bar sorptive extraction (SBSE)

This work presents the development, optimization and validation of a multi-residue method for the simultaneous determination of 102 contaminants, including fragrances, UV filters, repellents, endocrine disruptors, biocides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and several types of pesticides in aqueous matrices. Water samples were processed using stir bar sorptive extraction (SBSE) after the optimization of several parameters: agitation time, ionic strength, presence of organic modifiers, pH, and volume of the derivatizing agent. Target compounds were extracted from the bars by liquid desorption (LD). Separation, identification and quantification of analytes were carried out by gas chromatography (GC) coupled to time-of-flight (ToF-MS) mass spectrometry. A new ionization source, atmospheric pressure gas chromatography (APGC), was tested. The optimized protocol showed acceptable recovery percentages (50–100%) and limits of detection below 1 ng L−1 for most of the compounds. Occurrence of 21 out of 102 analytes was confirmed in several environmental aquatic matrices, including seawater, sewage effluent, river water and groundwater. Non-target compounds such as organophosphorus flame retardants were also identified in real samples by accurate mass measurement of their molecular ions using GC-APGC–ToF-MS. To the best of our knowledge, this is the first time that this technique has been applied for the analysis of contaminants in aquatic systems. By employing lower energy than the more widely used electron impact ionization (EI), AGPC provides significant advantages over EI for those substances very susceptible to high fragmentation (e.g., fragrances, pyrethroids)

Quasiparticle vanishing driven by geometrical frustration

We investigate the single hole dynamics in the triangular t-J model. We study the structure of the hole spectral function, assuming the existence of a 120 magnetic Neel order. Within the self-consistent Born approximation (SCBA) there is a strong momentum and t sign dependence of the spectra, related to the underlying magnetic structure and the particle-hole asymmetry of the model. For positive t, and in the strong coupling regime, we find that the low energy quasiparticle excitations vanish outside the neighbourhood of the magnetic Goldstone modes; while for negative t the quasiparticle excitations are always well defined. In the latter, we also find resonances of magnetic origin whose energies scale as (J/t)^2/3 and can be identified with string excitations. We argue that this complex structure of the spectra is due to the subtle interplay between magnon-assisted and free hopping mechanisms. Our predictions are supported by an excellent agreement between the SCBA and the exact results on finite size clusters. We conclude that the conventional quasiparticle picture can be broken by the effect of geometrical magnetic frustration.Comment: 6 pages, 7 figures. Published versio

Active faulting and earthquakes in the central Alboran Sea

Central Alboran Sea constitutes a key area to analyze the relationships between active tectonic structures and moderate seismicity. The heterogeneous crustal layered structure and the propagation of the deformation along the fault zones are key features to relate active faults and seismic hazard. The NW-SE oblique convergence between Eurasian and African plates determines the broad band of tectonic deformation and seismicity along the Alboran Sea basin (westernmost Mediterranean). The Betic-Rif Cordilleras are connected through the Gibraltar Arc and surround the Alboran Sea formed by thinned continental crust. This basin is filled since the Neogene by sedimentary and volcanic rocks. Central Alboran Sea is now undergoing shortening and is mainly deformed by a system of conjugated WNW-ESE dextral and NE-SW sinistral faults with recent activity formed by indenter tectonics related to a heterogeneous crustal behavior. In addition a NNW-SSE normal fault set and large ENE-WSW folds deform the central and northern Alboran Sea. These structures support a present-day N160 E maximum compression and orthogonal extension. In this setting, most of the recent seismicity is concentrated along a NNE-SSW zone that extends landward from Campo de Dalias (SE Spain) to Al Hoceima (Morocco). The Campo de Dalias 1993-1994 seismic crisis reached up to magnitudes Mw= 5.3, while the Al Hoceima area was affected by seismic crisis of 1994 (May 26, Mw=5.6) and 2004 (Feb 24, Mw= 6.4). The main active fault related to the 2004 earthquakes, even that it was a vertical NNE-SSW oriented fault and focal depth was 6 km, did not reach the surface probably due to the presence of a mechanically layered crust. The main active surface faults located in Al Hoceima area (the NNE-SSW transtensional sinistral Trougout fault zone) extended northwards towards the NE-SW sinistral Al Idrissi Fault that intersects the Alboran Sea and are connected with the normal NNW-SSE Balanegra Fault zone. Although these active faults determine most of the seismicity of the central Alboran Sea, the seismic crisis that occurred since January 2016, reaching Mw= 6.3 is located in a region westward of Al Idrissi Fault, underlining the relevance of growth of new faults to determine the seismic hazard of the region. Earthquake focal mechanisms support that the main active fault has a NNE-SSW orientation, similar to Al Idrissi Fault zone. Maximum magnitude suggests a surface rupture length of at least 12 km and a subsurface rupture of 20 km. Propagation of a new fault is more efficient to accumulate elastic deformation, and to produce highest magnitude earthquakes than already formed faults. This new fault is connected probably in deep crustal levels with the blind sinistral fault responsible of the 2004 Al Hoceima earthquake. The tsunami hazard of the region should be consequence of both seabottom displacement due to fault activity and co-seismic submarine landslides. The INCRISIS cruise, scheduled by May 2016, will provide evidences of seabottom effects of this seismic crisis

Silver nanoparticle embedded copper oxide as an efficient core–shell for the catalytic reduction of 4-nitrophenol and antibacterial activity improvement

International audienc

Magnetic properties of the quantum spin-1/2 XX diamond chain: The Jordan-Wigner approach

The Jordan-Wigner transformation is applied to study magnetic properties of the quantum spin-1/2 $XX$ model on the diamond chain. Generally, the Hamiltonian of this quantum spin system can be represented in terms of spinless fermions in the presence of a gauge field and different gauge-invariant ways of assigning the spin-fermion transformation are considered. Additionally, we analyze general properties of a free-fermion chain, where all gauge terms are neglected and discuss their relevance for the quantum spin system. A consideration of interaction terms in the fermionic Hamiltonian rests upon the Hartree-Fock procedure after fixing the appropriate gauge. Finally, we discuss the magnetic properties of this quantum spin model at zero as well as non-zero temperatures and analyze the validity of the approximation used through a comparison with the results of the exact diagonalization method for finite (up to 36 spins) chains. Besides the $m=1/3$ plateau the most prominent feature of the magnetization curve is a jump at intermediate field present for certain values of the frustrating vertical bond.Comment: 12 pages, 9 figures, accepted for publication in Eur. Phys. J.

Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in α′\alpha'-NaV2_2O5_5 studied by far infrared spectroscopy

We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3 and 200cm-1 in polarizations of incident light parallel to a, b, and c crystallographic axes in magnetic fields up to 33T. The triplet origin of an excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The magnitude of the spin gap at low temperatures is found to be magnetic field independent at least up to 33T. All other infrared-active transitions appearing below Tc are ascribed to zone-folded phonons. Two different dynamic Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to the oscillator strength of the otherwise forbidden singlet to triplet transition. 1. The strongest singlet to triplet transition is an electric dipole transition where the polarization of the incident light's electric field is parallel to the ladder rungs, and is allowed by the dynamic DM interaction created by a high frequency optical a-axis phonon. 2. In the incident light polarization perpendicular to the ladder planes an enhancement of the singlet to triplet transition is observed when the applied magnetic field shifts the singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon energy. The origin of this mechanism is the dynamic DM interaction created by the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had been los

Jordan-Wigner approach to dynamic correlations in spin-ladders

We present a method for studying the excitations of low-dimensional quantum spin systems based on the Jordan-Wigner transformation. Using an extended RPA-scheme we calculate the correlation function of neighboring spin flips which well approximates the optical conductivity of ${\rm Sr_2CuO_3}$. We extend this approach to the two-leg $S=1/2$--ladder by numbering the spin operators in a meander-like sequence. We obtain good agreement with the optical conductivity of the spin ladder compound (La,Ca)$_{14}$Cu$_{24}$O$_{41}$ for polarization along the rungs. For polarization along the legs higher order correlations are important to explain the weight of high-energy continuum excitations and we estimate the contribution of 4-- and 6--fermion processes.Comment: 15 pages, 16 figure