arXiv subjects

Roser Valenti

Publications and source records attributed to Roser Valenti.

At least 55 records · Page 3Linked to original sources

arXiv2019

Kitaev honeycomb models in magnetic fields: Dynamical response and hidden symmetries

Motivated by recent reports of a field-induced intermediate phase (IP) in the antiferromagnetic honeycomb Kitaev model that may be a spin liquid whose nature is distinct from the Kitaev Z2 phase, we present a detailed numerical study on the nature and dynamical response (such as dynamical spin-structure factors and resonant inelastic x-ray scattering intensities) of this field-induced IP and neighboring phases in a family of Kitaev-based models related by hidden symmetries and duality transformations. We further show that the same field-induced IP can appear in models relevant for $α$-RuCl${_3}$, which exhibit a ferromagnetic Kitaev coupling and additional interactions. In $α$-RuCl${_3}$, the IP represents a new phase, that is likely independent from the putative field-induced (spin-liquid) phase recently reported from thermal Hall conductivity measurements.

cond-mat.str-el↗

arXiv2019

Critical Spin Liquid versus Valence Bond Glass in Triangular Lattice Organic $κ$-(ET)$_2$Cu$_2$(CN)$_3$

In the quest for materials with unconventional quantum phases, the organic triangular-lattice antiferromagnet $κ$-(ET)$_2$Cu$_2$(CN)$_3$ has been extensively discussed as a quantum spin liquid (QSL) candidate. Recently, an intriguing quantum critical behaviour was suggested from low-temperature magnetic torque experiments. Through microscopic analysis of all anisotropic contributions, including Dzyaloshinskii-Moriya and multi-spin scalar chiral interactions, we highlight significant deviations of the experimental observations from a quantum critical scenario. Instead, we show that disorder-induced spin defects provide a comprehensive explanation of the low-temperature properties. These spins are attributed to valence bond defects that emerge spontaneously as the QSL enters a valence bond glass phase at low temperature. This theoretical treatment is applicable to a general class of frustrated magnetic systems and has important implications for the interpretation of magnetic torque, nuclear magnetic resonance, thermal transport and thermodynamic experiments.

cond-mat.str-el↗

arXiv2019

On the saw-tooth torque in anisotropic $j_{\rm eff} = 1/2$ magnets: Application to $α$-RuCl$_3$

The so-called "Kitaev candidate" materials based on $4d^5$ and $5d^5$ metals have recently emerged as magnetic systems displaying strongly anisotropic exchange interactions reminiscent of the Kitaev's honeycomb model. Recently, these materials have been shown to commonly display a distinct saw-tooth angular dependence of the magnetic torque over a wide range of magnetic fields. While higher order chiral spin interactions have been considered as a source of this observation, we show here that bilinear anisotropic interactions and/or $g$-anisotropy are each sufficient to explain the observed torque response, which may be distinguished on the basis of high-field measurements. These findings unify the understanding of magnetic torque experiments in a variety of Kitaev candidate materials.

cond-mat.str-el↗

arXiv2018

$ab$-$initio$ Approaches for Low-Energy Spin Hamiltonians

Implicit in the study of magnetic materials is the concept of spin Hamiltonians, which emerge as the low-energy theories of correlation-driven insulators. In order to predict and establish such Hamiltonians for real materials, a variety of first principles $ab$-$initio$ methods have been developed, based on density functional theory and wavefunction methodologies. In this review, we provide a basic introduction to such methods and the essential concepts of low-energy Hamiltonians, with a focus on their practical capabilities and limitations. We further discuss our recent efforts toward understanding a variety of complex magnetic systems that present unique challenges from the perspective of $ab$-$initio$ approaches.

cond-mat.str-el↗

arXiv2018

Role of Hydrogen in the spin-orbital-entangled quantum liquid candidate H$_3$LiIr$_2$O$_6$

Motivated by recent reports of H$_3$LiIr$_2$O$_6$ as a spin-orbital-entangled quantum liquid, we investigate via a combination of density functional theory and non-perturbative exact diagonalization the microscopic nature of its magnetic interactions. We find that while the interlayer O-H-O bond geometry strongly affects the local magnetic couplings, these bonds are likely to remain symmetrical due to large zero-point fluctuations of the H positions. In this case, the estimated magnetic model lies close to the classical tricritical point between ferromagnetic, zigzag and incommensurate spiral orders, what may contribute to the lack of magnetic ordering. However, we also find that substitution of H by D (deuterium) as well as disorder-induced inhomogeneities destabilize the O-H/D-O bonds modifying strongly the local magnetic couplings. These results suggest that the magnetic response in H$_3$LiIr$_2$O$_6$ is likely sensitive to both the stoichiometry and microstructure of the samples and emphasize the importance of a careful treatment of hydrogen for similar systems.

cond-mat.str-el↗

arXiv2018

Evidence for electronically-driven ferroelectricity in the family of strongly correlated dimerized BEDT-TTF molecular conductors

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge order. Our results indicate that intra-dimer charge degrees of freedom are of particular importance in correlated $κ$-(BEDT-TTF)$_2$X salts and can create novel states, such as electronically-driven multiferroicity or charge-order-induced quasi-1D spin liquids.

cond-mat.str-el↗

arXiv2018

Self-Energy Dispersion in the Hubbard Model

We introduce the concept of self-energy dispersion as an error bound on local theories and apply it to the two-dimensional Hubbard model on the square lattice at half-filling. Since the self-energy has no single-particle analog and is not directly measurable in experiments, its general behavior as a function of momentum is an open question. In this article we benchmark the momentum dependence with the Two-Particle Self-Consistent approach together with analytical and numerical considerations and we show that through the addition of a local single-particle potential to the Hubbard model the self-energy can be flattened, such that it is essentially described by only a frequency-dependent term. We use this observation to motivate that local theories, such as the dynamical mean-field theory, should be expected to give very accurate results in the presence of a potential of this kind. Finally, we propose a simple energy argument as an estimator for the crossover from non-local to local self-energies, which can be computed even by local theories such as dynamical mean-field theory.

cond-mat.str-el↗

arXiv2018

Trends in pressure-induced layer-selective half-collapsed tetragonal phases in the iron-based superconductor family $Ae{}A$Fe$_4$As$_4$

By performing pressure simulations within density functional theory for the family of iron-based superconductors $Ae{}A$Fe$_4$As$_4$ with $Ae$ = Ca, Sr, Ba and $A$ = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures $P_{c_1}$ and $P_{c_2}$, which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe$_4$As$_4$, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe$_4$As$_4$ and EuCsFe$_4$As$_4$, where Eu magnetism coexists with superconductivity, are discussed as well in the context of half-collapsed phases.

cond-mat.mtrl-sci↗

arXiv2018

Dynamics and fragmentation mechanism of (CH3-C5H4)Pt(CH3)3 on SiO2 Surfaces

The interaction of (CH3-C5H4)Pt(CH3)3 ((methylcyclopentadienyl)trimethylplatinum)) molecules on fully and partially hydroxylated SiO2 surfaces, as well as the dynamics of this interaction were investigated using density functional theory (DFT) and finite temperature DFT-based molecular dynamics simulations. Fully and partially hydroxylated surfaces represent substrates before and after electron beam treatment and this study examines the role of electron beam pretreatment on the substrates in the initial stages of precursor dissociation and formation of Pt deposits. Our simulations show that on fully hydroxylated surfaces or untreated surfaces, the precursor molecules remain inactivated while we observe fragmentation of (CH3-C5H4)Pt(CH3)3 on partially hydroxylated surfaces. The behavior of precursor molecules on the partially hydroxylated surfaces has been found to depend on the initial orientation of the molecule and the distribution of surface active sites. Based on the observations from the simulations and available experiments, we discuss possible dissociation channels of the precursor.

cond-mat.mtrl-sci↗

arXiv2018

Probing $α$-RuCl$_3$ Beyond Magnetic Order: Effects of Temperature and Magnetic Field

Recent studies have brought $α$-RuCl$_3$ to the forefront of experimental searches for materials realizing Kitaev spin-liquid physics. This material exhibits strongly anisotropic exchange interactions afforded by the spin-orbit coupling of the 4d Ru centers. We investigate the dynamical response at finite temperature and magnetic field for a realistic model of the magnetic interactions in $α$-RuCl$_3$. These regimes are thought to host unconventional paramagnetic states that emerge from the suppression of magnetic order. Using exact diagonalization calculations of the quantum model complemented by semi-classical analysis, we find a very rich evolution of the spin dynamics as the applied field suppresses the zigzag order and stabilizes a quantum paramagnetic state that is adiabatically connected to the fully polarized state at high fields. At finite temperature, we observe large redistributions of spectral weight that can be attributed to the anisotropic frustration of the model. These results are compared to recent experiments, and provide a roadmap for further studies of these regimes.

cond-mat.str-el↗

arXiv2018

Superconductivity in correlated BEDT-TTF molecular conductors: critical temperatures and gap symmetries

Starting from an {\it ab initio}-derived two-site dimer Hubbard hamiltonian on a triangular lattice, we calculate the superconducting gap functions and critical temperatures for representative $κ$-(BEDT-TTF)$_2$X superconductors by solving the linearized Eliashberg equation using the Two-Particle Self-Consistent approach (TPSC) extended to multi-site problems. Such an extension allows for the inclusion of molecule degrees of freedom in the description of these systems. We present both, benchmarking results for the half-filled dimer model as well as detailed investigations for the 3/4-filled molecule model. Remarkably, we find in the latter model that the phase boundary between the two most competing gap symmetries discussed in the context of these materials -d$_{xy}$ and the recently proposed eight-node gap s+d$_{x^2-y^2}$ symmetry- is located within the regime of realistic model parameters and is especially sensitive to the degree of in-plane anisotropy in the materials as well as to the value of the on-site Hubbard repulsion. We show that these results provide a more complete and accurate description of the superconducting properties of $κ$-(BEDT-TTF)$_2$X than previous Random Phase Approximation (RPA) calculations and, in particular, we discuss predicted critical temperatures in comparison to experiments. Finally, our findings suggest that it may be even easier to experimentally switch between the two pairing symmetries as previously anticipated by invoking pressure, chemical doping or disorder effects.

cond-mat.supr-con↗

arXiv2017

Models and Materials for Generalized Kitaev Magnetism

The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by G. Jackeli and G. Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na$_2$IrO$_3$, $α$-Li$_2$IrO$_3$, and $α$-RuCl$_3$), three-dimensional Kitaev materials ($β$- and $γ$-Li$_2$IrO$_3$), and other potential candidates, completing the review with the list of open questions awaiting new insights.

cond-mat.str-el↗

arXiv2017

Breakdown of Magnons in a Strongly Spin-Orbital Coupled Magnet

The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic magnons - quantized harmonic fluctuations of the ordered spins. In striking contrast is the recent discovery that strongly spin-orbital coupled magnets, such as $α$-RuCl$_3$, may display a broad excitation continuum inconsistent with conventional magnons. Due to incomplete knowledge of the underlying interactions unraveling the nature of this continuum remains challenging. The most discussed explanation refers to a coherent continuum of fractional excitations analogous to the celebrated Kitaev spin liquid. Here we present a more general scenario. We propose that the observed continuum represents incoherent excitations originating from strong magnetic anharmoniticity that naturally occurs in such materials. This scenario fully explains the observed inelastic magnetic response of $α$-RuCl$_3$ and reveals the presence of nontrivial excitations in such materials extending well beyond the Kitaev state.

cond-mat.str-el↗

arXiv2017

Cu-Sb dumbbell arrangement in the spin-orbital liquid candidate Ba$_3$CuSb$_2$O$_9$

The absence of both spin freezing and of a static Jahn-Teller effect have lead to the proposition that Ba$_3$CuSb$_2$O$_9$ is a quantum spin-orbital liquid. However, theoretical understanding of the microscopic origin of this behavior has been hampered by a lack of consensus on the lattice structure. Cu ions have been proposed to realize either a triangular lattice, a short-range ordered honeycomb lattice or a disordered lattice with stripelike correlations. Here we analyze the stability of idealized versions of these arrangements using density functional theory. We find stripe order of Cu ions to be energetically favored, hinting towards stripelike local Cu-Cu arrangements, while long-range order is presumably hindered due to disorder effects. Furthermore, we find evidence of significant interlayer interactions between Cu-Sb dumbbells, which affects the out-of-plane arrangement. Analysis of the relaxed crystal structures, electronic properties and tight-binding parameters provides clues as to the nature of the Jahn-Teller distortions.

cond-mat.str-el↗

arXiv2017

Basic electronic properties of iron selenide under variation of structural parameters

Since the discovery of high-temperature superconductivity in the thin-film FeSe/SrTiO$_3$ system, iron selenide and its derivates have been intensively scrutinized. Using ab initio density functional theory calculations we review the electronic structures that could be realized in iron-selenide if the structural parameters could be tuned at liberty. We calculate the momentum-dependence of the susceptibility and investigate the symmetry of electron pairing within the random phase approximation. Both the susceptibility and the symmetry of electron pairing depend on the structural parameters in a nontrivial way. These results are consistent with the known experimental behavior of binary iron chalcogenides and, at the same time, reveal two promising new ways of tuning superconducting transition temperatures in these materials. On the one hand by expanding the iron lattice of FeSe at constant iron-selenium distance and, on the other hand, by increasing the iron-selenium distance with unchanged iron lattice.

cond-mat.supr-con↗

arXiv2017

Role of vertex corrections in the matrix formulation of the random phase approximation for the multiorbital Hubbard model

In the framework of a multiorbital Hubbard model description of superconductivity, a matrix formulation of the superconducting pairing interaction that has been widely used is designed to treat spin, charge and orbital fluctuations within a random phase approximation (RPA). In terms of Feynman diagrams, this takes into account particle-hole ladder and bubble contributions as expected. It turns out, however, that this matrix formulation also generates additional terms which have the diagrammatic structure of vertex corrections. Here we examine these terms and discuss the relationship between the matrix-RPA superconducting pairing interaction and the Feynman diagrams that it sums.

cond-mat.supr-con↗

arXiv2016

Interplay of nematic and magnetic orders in FeSe under pressure

We offer an explanation for the recently observed pressure-induced magnetic state in the iron-chalcogenide FeSe based on \textit{ab initio} estimates for the pressure evolution of the most important Coulomb interaction parameters. We find that an increase of pressure leads to an overall decrease mostly in the nearest-neighbor Coulomb repulsion, which in turn leads to a reduction of the nematic order and the generation of magnetic stripe order. We treat the concomitant effects of band renormalization and the induced interplay of nematic and magnetic order in a self-consistent way and determine the generic topology of the temperature-pressure phase diagram, and find qualitative agreement with the experimentally determined phase diagram.

cond-mat.supr-con↗

arXiv2016

Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe

We use angle-resolved photo-emission spectroscopy (ARPES) to explore the electronic structure of single crystals of FeSe over a wide range of binding energies and study the effects of strong electron-electron correlations. We provide evidence for the existence of "Hubbard-like bands" at high binding energies consisting of incoherent many-body excitations originating from Fe $3d$ states in addition to the renormalized quasiparticle bands near the Fermi level. Many high energy features of the observed ARPES data can be accounted for when incorporating effects of strong local Coulomb interactions in calculations of the spectral function via dynamical mean-field theory, including the formation of a Hubbard-like band. This shows that over the energy scale of several eV, local correlations arising from the on-site Coulomb repulsion and Hund's coupling are essential for a proper understanding of the electronic structure of FeSe and other related iron based superconductors.

cond-mat.supr-con↗