arXiv subjects

J. Merino

Publications and source records attributed to J. Merino.

At least 19 recordsLinked to original sources

arXiv2025

Characterisation of the plutonium isotopic composition of a sediment core from Palomares, Spain, by low-energy AMS and alpha-spectrometry

The measurement of plutonium isotopes, 239Pu and 240Pu, at 670 kV on the compact accelerator mass spectrometry (AMS) system at the Centro Nacional de Aceleradores (CNA) in Seville, Spain, is now a reality. In this work, we present first Pu AMS results for environmental samples: a sediment core collected in a submarine canyon in the Mediterranean coast of the Spanish region of Palomares, affected by a nuclear accident in 1966. From the study of the 240Pu/239Pu atomic ratio profile, showing on average levels lower than 11%, we confirm that the weapon-grade plutonium released on land during the accident, with a characteristic 240Pu/239Pu atomic ratio of 5.8%, has found its way into the marine environment. A two-plutonium sources mixture model (Palomares and fallout) is used to elucidate the percentage of the plutonium coming from the accident. As a validation exercise of the Pu AMS measuring technique and in order to obtain the 238Pu/(239+240)Pu activity ratios, samples were also studied by alpha-spectrometry (AS). The obtained AS 239+240Pu activity concentration results fit in with the AMS ones in a wide dynamic range, thus validating the AMS technique.

physics.ins-det↗

arXiv2018

Spin molecular-orbit coupling and magnetic properties of the decorated honeycomb layers of Mo3S7(dmit)3 crystals

We explore the magnetic properties of isolated a-b planes of trinuclear organometallic crystals, Mo3S7(dmit)3, in which an interplay of strong electronic correlations and spin molecular-orbital coupling (SMOC) occurs. The magnetic properties can be described by a XXZ+120, S=1 Heisenberg model on a honeycomb lattice with single-spin anisotropy, D, which depends strongly on SMOC. Based on ab initio estimates of SMOC in Mo3S7(dmit)3 crystals, we predict that the honeycomb layers of Mo3S7(dmit)3 are Neel ordered. However, in materials with a greater degree of magnetic frustration, Neel order can give way to the large-D phase.

cond-mat.str-el↗

arXiv2018

Role of quantum fluctuations on spin liquids and ordered phases in the Heisenberg model on the honeycomb lattice

Motivated by the rich physics of honeycomb magnetic materials, we obtain the phase diagram and analyze magnetic properties of the spin-1/2 and spin-1 J1-J2-J3 Heisenberg model on the honeycomb lattice. Based on the SU(2) and SU(3) symmetry representations of the Schwinger boson approach, which treats disordered spin liquids and magnetically ordered phases on an equal footing, we obtain the complete phase diagrams in the (J2,J3)plane. This is achieved using a fully unrestricted approach which does not assume any pre-defined Ansatze. For S=1/2, we find a quantum spin liquid (QSL) stabilized between the N\'eel, spiral and collinear antiferromagnetic phases in agreement with previous theoretical work. However, by increasing S from 1/2 to 1, the QSL is quickly destroyed due to the weakening of quantum fluctuations indicating that the model already behaves as a quasi-classical system. The dynamical structure factors and temperature dependence of the magnetic susceptibility are obtained in order to characterize all phases in the phase diagrams. Moreover, motivated by the relevance of the single-ion anisotropy, D, to various S=1 honeycomb compounds, we have analyzed the destruction of magnetic order based on a SU(3) representation of the Schwinger bosons. Our analysis provides a unified understanding of the magnetic properties of honeycomb materials realizing the J1-J2-J3 Heisenberg model from the strong quantum spin regime at S=1/2 to the S=1 case. Neutron scattering and magnetic susceptibility experiments can be used to test the destruction of the QSL phase when replacing S=1/2 by S=1 localized moments in certain honeycomb compounds.

cond-mat.str-el↗

arXiv2017

Complementary views on electron spectra: From Fluctuation Diagnostics to real space correlations

We study the relation between the microscopic properties of a many-body system and the electron spectra, experimentally accessible by photoemission. In a recent paper [Phys. Rev. Lett. 114, 236402 (2015)], we introduced the "fluctuation diagnostics" approach, to extract the dominant wave vector dependent bosonic fluctuations from the electronic self-energy. Here, we first reformulate the theory in terms of fermionic modes, to render its connection with resonance valence bond (RVB) fluctuations more transparent. Secondly, by using a large-U expansion, where U is the Coulomb interaction, we relate the fluctuations to real space correlations. Therefore, it becomes possible to study how electron spectra are related to charge, spin, superconductivity and RVB-like real space correlations, broadening the analysis of an earlier work [Phys. Rev. B 89, 245130 (2014)]. This formalism is applied to the pseudogap physics of the two-dimensional Hubbard model, studied in the dynamical cluster approximation. We perform calculations for embedded clusters with up to 32 sites, having three inequivalent K-points at the Fermi surface. We find that as U is increased, correlation functions gradually attain values consistent with an RVB state. This first happens for correlation functions involving the antinodal point and gradually spreads to the nodal point along the Fermi surface. Simultaneously a pseudogap opens up along the Fermi surface. We relate this to a crossover from a Kondo-like state to an RVB-like localized cluster state and to the presence of RVB and spin fluctuations. These changes are caused by a strong momentum dependence in the cluster bath-couplings along the Fermi surface. We also show, from a more algorithmic perspective, how the time-consuming calculations in fluctuation diagnostics can be drastically simplified.

cond-mat.str-el↗

arXiv2017

Dynamical reduction of the dimensionality of exchange interactions and the "spin-liquid" phase of $\kappa$-(BEDT-TTF)$_2X$

We show that the anisotropy of the effective spin model for the dimer Mott insulator phase of $\kappa$-(BEDT-TTF)$_2X$ salts is dramatically different from that of the underlying tight-binding model. Intra-dimer quantum interference results in a model of coupled spin chains, where frustrated interchain interactions suppress long-range magnetic order. Thus, we argue, the "spin liquid" phase observed in some of these materials is a remnant of the Tomonaga-Luttinger physics of a single chain. This is consistent with previous experiments and resolves some outstanding puzzles. An erratum [Phys. Rev. Lett. 120, 199901 (2018).] is added as an appendix.

cond-mat.str-el↗

arXiv2017

Effects of anisotropy in spin molecular-orbital coupling on effective spin models of trinuclear organometallic complexes

We consider layered decorated honeycomb lattices at two-thirds filling, as realized in some trinuclear organometallic complexes. Localized $S=1$ moments with a single-spin anisotropy emerge from the interplay of Coulomb repulsion and spin molecular-orbit coupling (SMOC). Magnetic anisotropies with bond dependent exchange couplings occur in the honeycomb layers when the direct intracluster exchange and the spin molecular-orbital coupling are both present. We find that the effective spin exchange model within the layers is an XXZ + 120$^\circ$ honeycomb quantum compass model. The intrinsic non-spherical symmetry of the multinuclear complexes leads to very different transverse and longitudinal spin molecular-orbital couplings, which greatly enhances the single-spin and exchange coupling anisotropies. The interlayer coupling is described by a XXZ model with anisotropic biquadratic terms. As the correlation strength increases the systems becomes increasingly one-dimensional. Thus, if the ratio of SMOC to the interlayer hopping is small this stabilizes the Haldane phase. However, as the ratio increases there is a quantum phase transition to the topologically trivial `$D$-phase'. We also predict a quantum phase transition from a Haldane phase to a magnetically ordered phase at sufficiently strong external magnetic fields.

cond-mat.str-el↗

arXiv2016

Heisenberg and Dzyaloshinskii-Moriya interactions controlled by molecular packing in tri-nuclear organometallic clusters

Motivated by recent synthetic and theoretical progress we consider magnetism in crystals of multi-nuclear organometallic complexes. We calculate the Heisenberg symmetric exchange and the Dzyaloshinskii-Moriya antisymmetric exchange. We show how, in the absence of spin-orbit coupling, the interplay of electronic correlations and quantum interference leads to a quasi-one dimensional effective spin model in a typical tri-nuclear complex, Mo$_3$S$_7$(dmit)$_3$, despite its underlying three dimensional band structure. We show that both intra- and inter-molecular spin-orbit coupling can cause an effective Dzyaloshinskii-Moriya interaction. Furthermore, we show that, even for an isolated pair of molecules the relative orientation of the molecules controls the nature of the Dzyaloshinskii-Moriya coupling. We show that interference effects also play a crucial role in determining the Dzyaloshinskii-Moriya interaction. Thus, we argue, that multi-nuclear organometallic complexes represent an ideal platform to investigate the effects of Dzyaloshinskii-Moriya interactions on quantum magnets.

cond-mat.str-el↗

arXiv2016

Spin-orbit coupling in {Mo$_3$S$_7$(dmit)$_3$}

Spin-orbit coupling in crystals is known to lead to unusual direction dependent exchange interactions, however understanding of the consequeces of such effects in molecular crystals is incomplete. Here we perform four component relativistic density functional theory computations on the multi-nuclear molecular crystal {Mo$_3$S$_7$(dmit)$_3$} and show that both intra- and inter-molecular spin-orbit coupling are significant. We determine a long-range relativistic single electron Hamiltonian from first principles by constructing Wannier spin-orbitals. We analyse the various contributions through the lens of group theory. Intermolecular spin-orbit couplings like those found here are known to lead to quantum spin-Hall and topological insulator phases on the 2D lattice formed by the tight-binding model predicted for a single layer of {Mo$_3$S$_7$(dmit)$_3$}.

cond-mat.str-el↗

arXiv2016

Topological quantum phase transition driven by anisotropic spin-orbit coupling in trinuclear organometallic coordination crystals

We show how quasi-one-dimensional correlated insulating states arise at two-thirds filling in organometallic multinuclear coordination complexes described by layered decorated honeycomb lattices. The interplay of spin-orbit coupling and electronic correlations leads to pseudospin-1 moments arranged in weakly coupled chains with highly anisotropic exchange and a large trigonal splitting. This leads to a quantum phase transition from a Haldane phase to a topologically trivial phase as the relative strength of the spin-orbit coupling increases.

cond-mat.str-el↗

arXiv2016

Spin-orbit coupling and strong electronic correlations in cyclic molecules

In atoms spin-orbit coupling (SOC) cannot raise the angular momentum above a maximum value or lower it below a minimum. Here we show that this need not be the case in materials built from nanoscale structures including multi-nuclear coordination complexes, materials with decorated lattices, or atoms on surfaces. In such cyclic molecules the electronic spin couples to currents running around the molecule. For odd-fold symmetric molecules (e.g., odd membered rings) the SOC is highly analogous to the atomic case; but for even-fold symmetric molecules every angular momentum state can be both raised and lowered. These differences arise because for odd-fold symmetric molecules the maximum and minimum molecular orbital angular momentum states are time reversal conjugates, whereas for even-fold symmetric molecules they are aliases of the same single state. We show, from first principles calculations, that in suitable molecules this molecular SOC is large, compared to the energy differences between frontier molecular orbitals. Finally, we show that, when electronic correlations are strong, molecular SOC can cause highly anisotropic exchange interactions and discuss how this can lead to effective spin models with compass Hamiltonians.

cond-mat.str-el↗

arXiv2016

Parquet decomposition calculations of the electronic self-energy

The parquet decomposition of the self-energy into classes of diagrams, those associated with specific scattering processes, can be exploited for different scopes. In this work, the parquet decomposition is used to unravel the underlying physics of non-perturbative numerical calculations. We show the specific example of dynamical mean field theory (DMFT) and its cluster extensions (DCA) applied to the Hubbard model at half-filling and with hole doping: These techniques allow for a simultaneous determination of two-particle vertex functions and self-energies, and hence, for an essentially "exact" parquet decomposition at the single-site or at the cluster level. Our calculations show that the self-energies in the underdoped regime are dominated by spin scattering processes, consistent with the conclusions obtained by means of the fluctuation diagnostics approach [Phys. Rev. Lett. 114, 236402 (2015)]. However, differently from the latter approach, the parquet procedure displays important changes with increasing interaction: Even for relatively moderate couplings, well before the Mott transition, singularities appear in different terms, with the notable exception of the predominant spin-channel. We explain precisely how these singularities, which partly limit the utility of the parquet decomposition, and - more generally - of parquet-based algorithms, are never found in the fluctuation diagnostics procedure. Finally, by a more refined analysis, we link the occurrence of the parquet singularities in our calculations to a progressive suppression of charge fluctuations and the formation of an RVB state, which are typical hallmarks of a pseudogap state in DCA.

cond-mat.str-el↗

arXiv2015

Self-energy enhancements in doped Mott insulators

We analyze enhancements in the magnitude of the self-energy for electrons far away from the Fermi surface in doped Mott insulators using the dynamical cluster approximation to the Hubbard model. For large onsite repulsion, U, and hole doping, the magnitude of the self-energy for imaginary frequencies at the top of the band (k=(pi,pi) is enhanced with respect to the self-energy magnitude at the bottom of the band (k=(0,0)). The self-energy behavior at these two k-points is switched for electron doping. Although the hybridization is much larger for (0,0) than for (pi,pi), we demonstrate that this is not the origin of this difference. Isolated clusters under a downward shift of the chemical potential, mu<U/2, at half-filling reproduce the overall self-energy behavior at (0,0) and (pi,pi) found in low hole doped embedded clusters. This happens although there is no change in the electronic structure of the isolated clusters. Our analysis shows that a downward shift of the chemical potential which weakly hole dopes the Mott insulator can lead to a large enhancement of the (pi,pi) self-energy which is not necessarily associated with electronic correlation effects, even in embedded clusters.

cond-mat.str-el↗

arXiv2014

Fluctuation diagnostics of the electron self-energy: Origin of the pseudogap physics

We demonstrate how to identify which physical processes dominate the low-energy spectral functions of correlated electron systems. We obtain an unambiguous classification through an analysis of the equation of motion for the electron self-energy in its charge, spin and particle-particle representations. Our procedure is then employed to clarify the controversial physics responsible for the appearance of the pseudogap in correlated systems. We illustrate our method by examining the attractive and repulsive Hubbard model in two-dimensions. In the latter, spin fluctuations are identified as the origin of the pseudogap, and we also explain why $d-$wave pairing fluctuations play a marginal role in suppressing the low-energy spectral weight, independent of their actual strength.

cond-mat.str-el↗

arXiv2014

Topological spin liquid phase in a low-dimensional organic molecular compound

We report the discovery of a correlated insulator with a bulk gap at two-thirds filling in a geometrically frustrated Hubbard model that describes the low-energy physics of Mo$_3$S$_7$(dmit)$_3$. This is very different from the Mott insulator expected at half-filling. We show that the insulating phase, which persists even for very weak electron-electron interactions ($U$), is adiabatically connected to the Haldane phase and is consistent with experiments on Mo$_3$S$_7$(dmit)$_3$

cond-mat.str-el↗

arXiv2014

Charge fluctuations in the unconventional metallic state of Li0.9Mo6O17

Charge fluctuations in the quasi-one-dimensional material Li0.9Mo6O17 are analyzed based on a multi orbital extended Hubbard model. A charge ordering transition induced by Coulomb repulsion is found with a charge ordering pattern different from a conventional charge density wave driven by Fermi surface nesting. The metallic state displays a characteristic charge collective mode which softens signalling the proximity to the transition. We argue that the strong scattering between electrons generated by these charge order fluctuations can lead to the unconventional metallic state observed above the superconducting transition temperature in Li0.9Mo6O17.

cond-mat.str-el↗

arXiv2014

Low-energy effective theories of the two-thirds filled Hubbard model on the triangular necklace lattice

Motivated by Mo$_3$S$_7$(dmit)$_3$, we investigate the Hubbard model on the triangular necklace lattice at two-thirds filling. We show, using second order perturbation theory, that in the molecular limit, the ground state and the low energy excitations of this model are identical to those of the spin-one Heisenberg chain. The latter model is known to be in the symmetry protected topological Haldane phase. Away from this limit we show, on the basis of symmetry arguments and density matrix renormalization group (DMRG) calculations, that the low-energy physics of the Hubbard model on the triangular necklace lattice at two-thirds filling is captured by the ferromagnetic Hubbard-Kondo lattice chain at half filling. This is consistent with and strengthens previous claims that both the half-filled ferromagnetic Kondo lattice model and the two-thirds filled Hubbard model on the triangular necklace lattice are also in the Haldane phase. A connection between Hund's rules and Nagaoka's theorem is also discussed.

cond-mat.str-el↗

arXiv2013

Pseudogap and singlet formation in cuprate and organic superconductors

The pseudogap phase occurring in cuprate and organic superconductors is analyzed based on the dynamical cluster approximation (DCA) approach to the Hubbard model. A cluster embedded in a self-consistent bath is studied. With increasing Coulomb repulsion, U, the antinodal point [k=(pi,0)] displays a gradual suppression of spectral density of states around the Fermi energy which is not observed at the nodal point [k=(pi/2,pi/2)]. The opening of the antinodal pseudogap is related to the internal structure of the cluster and the much weaker bath-cluster couplings at the antinodal than nodal point. The role played by internal cluster correlations is elucidated from a simple four-level model. For small U, the cluster levels form Kondo singlets with their baths leading to a peak in the spectral density. As U is increased a localized state is formed localizing the electrons in the cluster. If this cluster localized state is non-degenerate, the Kondo effect is destroyed and a pseudogap opens up in the spectra at the anti-nodal point. The pseudogap can be understood in terms of destructive interference between different paths for electrons hopping between the cluster and the bath. However, electrons at the nodal points remain in Kondo states up to larger U since they are more strongly coupled to the bath. The strong correlation between the (pi,0) and (0,pi) cluster levels in the localized state leads to a large correlation energy gain which is important for localizing electrons and opening up a pseudogap. Such scenario is in contrast with two independent Mott transitions found in two-band systems with different bandwidths in which the localized cluster electron does not correlate strongly with any other cluster electron for intermediate U. The important intracluster sector correlations are associated with the resonating valence bond (RVB) character of the cluster.

cond-mat.str-el↗

arXiv2012

Pseudogap in cuprate and organic superconductors

We study the pseudogap present in cuprate and organic superconductors. We use the dynamical cluster approximation (DCA), treating a cluster embedded in a bath. As the Coulomb interaction is increased, cluster-bath Kondo states are destroyed and bound cluster states formed. We show that this leads to a pseudogap. Due to weaker coupling to the bath for the anti-nodal point, this happens first for this point, explaining the k-dependence of the pseudogap. The pseudogap can be understood in terms of preformed d-wave pairs, but it does not prove the existence of such pairs.

cond-mat.str-el↗