arXiv subjects

Roser Valentí

Publications and source records attributed to Roser Valentí.

At least 55 records · Page 3Linked to original sources

arXiv2024

Bound states and local topological phase diagram of classical impurity spins coupled to a Chern insulator

The existence of bound states induced by local impurities coupled to an insulating host depends decisively on the global topological properties of the host's electronic structure. In this context, we consider magnetic impurities modelled as classical unit-length spins that are exchange-coupled to the spinful Haldane model on the honeycomb lattice. We investigate the spectral flow of bound states with the coupling strength $J$ in both the topologically trivial and Chern-insulating phases. In addition to conventional $k$-space topology, an additional, spatially local topological feature is available, based on the space of impurity-spin configurations forming, in case of $R$ impurities, an $R$-fold direct product of two-dimensional spheres. Global $k$-space and local $S$-space topology are represented by different topological invariants, the first ($k$-space) Chern number and the $R$-th ($S$-space) spin-Chern number. We demonstrate that there is a local $S$-space topological transition as a function of $J$ associated with a change in the spin Chern number and work out the implications of this for the $J$-dependent local electronic structure close to the impurities and, in particular, for in-gap bound states. The critical exchange couplings' dependence on the parameters of the Haldane model, and thus on the $k$-space topological state, is obtained numerically to construct local topological phase diagrams for systems with $R=1$ and $R=2$ impurity spins.

cond-mat.mes-hall↗

arXiv2024

Fragility of the magnetic order in the prototypical altermagnet RuO$_2$

Altermagnetism is a topic that has lately been gaining attention and the RuO$_2$ compound is among one of the most studied altermagnetic candidates. However, the survey of available literature on RuO$_2$ properties suggests that there is no consensus about the magnetism of this material. By performing density functional theory calculations, we show that the electronic properties of stoichiometric RuO$_2$ are described in terms of a smaller Hubbard $U$ within DFT+$U$ than the value required to have magnetism. We further argue that Ru vacancies can actually aid the formation of a magnetic state in RuO$_2$. This in turn suggests that a characterization of the amount of Ru vacancies in experimental samples might help the resolution of the controversy between the different experimental results.

cond-mat.mtrl-sci↗

arXiv2024

Spin Hall conductivity in the Kane-Mele-Hubbard model at finite temperature

The Kane-Mele model is known to show a quantized spin Hall conductivity at zero temperature. Including Hubbard interactions at each site leads to a quantum phase transition to an XY antiferromagnet at sufficiently high interaction strength. Here, we use the two-particle self-consistent approach (TPSC), which we extend to include spin-orbit coupling, to investigate the Kane-Mele-Hubbard model at finite temperature and half-filling. TPSC is a weak to intermediate coupling approach capable of calculating a frequency- and momentum-dependent self-energy from spin and charge fluctuations. We present results for the spin Hall conductivity and correlation lengths for antiferromagnetic spin fluctuations for different values of temperature, spin-orbit coupling and Hubbard interaction. The vertex corrections, which here are analogues of Maki-Thompson contributions, show a strong momentum dependence and give a large contribution in the vicinity of the phase transition at all temperatures. Their inclusion is necessary to observe the quantization of the spin Hall conductivity for the interacting system in the zero temperature limit. At finite temperature, increasing the Hubbard interaction leads to a decrease of the spin Hall conductivity. This decrease can be explained by band-gap renormalization from scattering of electrons on antiferromagnetic spin fluctuations.

cond-mat.str-el↗

arXiv2023

Magnetic properties of multi-domain epitaxial EuPd$_2$ thin films

Europium intermetallic compounds show a variety of different ground states and anomalous physical properties due to the interactions between the localized 4f electrons and the delocalized electronic states. Europium is also the most reactive of the rare earth metals which might be the reason why very few works are concerned with the properties of Eu-based thin films. Here we address the low-temperature magnetic properties of ferromagnetic EuPd$_2$ thin films prepared by molecular beam epitaxy. The epitaxial (111)-oriented thin films grow on MgO (100) with eight different domain orientations. We analyze the low-temperature magnetic hysteresis behavior by means of micromagnetic simulations taking the multi-domain morphology explicitly into account and quantify the magnetic crystal anisotropy contribution. By ab initio calculations we trace back the microscopic origin of the magnetic anisotropy to thin film-induced biaxial strain.

cond-mat.mtrl-sci↗

arXiv2023

Topology of SmB6 revisited by means of topological quantum chemistry

The mixed-valence compound SmB6 with partially filled samarium 4f flat bands hybridizing with 5d conduction bands is a paramount example of a correlated topological heavy-fermion system. In this study we revisit the topology of SmB6 with the band theory paradigm and uncover previously overlooked aspects resulting from the formation of multiple topological gaps in the electronic structure. By invoking topological quantum chemistry (TQC) we provide a detailed classification of the strong and crystalline topological features that derive from the existence of such topological gaps. To corroborate this classification, we calculate Wilson loops and simulate the surface electronic structure using a minimal tight-binding model, allowing us to describe its surface states and confirm the crystalline topology. We finally discuss its implications for experiments.

cond-mat.str-el↗

arXiv2023

Strain control of band topology and surface states in antiferromagnetic EuCd$_2$As$_2$

Topological semimetal antiferromagnets provide a rich source of exotic topological states which can be controlled by manipulating the orientation of the Néel vector, or by modulating the lattice parameters through strain. We investigate via ${ab\ initio}$ density functional theory calculations, the effects of shear strain on the bulk and surface states n two antiferromagnetic EuCd$_2$As$_2$ phases with out-of-plane and in-plane spin configurations. When magnetic moments are along the $\textit{c}$-axis, a $3\%$ longitudinal or diagonal shear strain can tune the Dirac semimetal phase to an axion insulator phase, characterized by the parity-based invariant $η_{4I} = 2$. For an in-plane magnetic order, the axion insulator phase remains robust under all shear strains. We further find that for both magnetic orders, the bulk gap increases and a surface gap opens on the (001) surface up to 16 meV. Because of a nonzero $η_{4I}$ index and gapped states on the (001) surface, hinge modes are expected to happen on the side surface states between those gapped surface states. This result can provide a valuable insight in the realization of the long-sought axion states.

cond-mat.str-el↗

arXiv2023

Constraints on the superconducting state of Sr$_2$RuO$_4$ from elastocaloric measurements

Strontium ruthenate Sr$_2$RuO$_4$ is an unconventional superconductor whose pairing symmetry has not been fully clarified, despite more than two decades of intensive research. Recent NMR Knight shift experiments have rekindled the Sr$_2$RuO$_4$ pairing debate by giving strong evidence against all odd-parity pairing states, including chiral $p$-wave pairing that was for a long time the leading pairing candidate. Here, we exclude additional pairing states by analyzing recent elastocaloric measurements [YS. Li et al., Nature 607, 276--280 (2022)]. To be able to explain the elastocaloric experiment, we find that unconventional even-parity pairings must include either large $d_{x^2 - y^2}$-wave or large $\{d_{xz} \mid d_{yz}\}$-wave admixtures, where the latter possibility arises because of the body-centered point group symmetry. These $\{d_{xz} \mid d_{yz}\}$-wave admixtures take the form of distinctively body-centered-periodic harmonics that have horizontal line nodes. Hence $g_{xy(x^2-y^2)}$-wave and $d_{xy}$-wave pairings are excluded as possible dominant even pairing states.

cond-mat.supr-con↗

arXiv2023

Tuning superconductivity and spin-vortex fluctuations in CaKFe$_4$As$_4$ through in-plane antisymmetric strains

Lattice strains of appropriate symmetry have served as an excellent tool to explore the interaction of superconductivity in the iron-based superconductors with nematic and stripe spin-density wave (SSDW) order, which are both closely tied to an orthorhombic distortion. In this work, we contribute to a broader understanding of the coupling of strain to superconductivity and competing normal-state orders by studying CaKFe$_4$As$_4$ under large, in-plane strains of $B_{1g}$ and $B_{2g}$ symmetry. In contrast to the majority of iron-based superconductors, pure CaKFe$_4$As$_4$ exhibits superconductivity with relatively high transition temperature of $T_c\,\sim\,$35 K in proximity of a non-collinear, tetragonal, hedgehog spin-vortex crystal (SVC) order. Through experiments, we demonstrate an anisotropic in-plane strain response of $T_c$, which is reminiscent of the behavior of other pnictides with nematicity. However, our calculations suggest that in CaKFe$_4$As$_4$, this anisotropic response correlates with the one of the SVC fluctuations, highlighting the close interrelation of magnetism and high-$T_c$ superconductivity. By suggesting moderate $B_{2g}$ strains as an effective parameter to change the stability of SVC and SSDW, we outline a pathway to a unified phase diagram of iron-based superconductivity.

cond-mat.supr-con↗

arXiv2023

Growth of self-integrated atomic quantum wires and junctions of a Mott semiconductor

Continued advances in quantum technologies rely on producing nanometer-scale wires. Although several state-of-the-art nanolithographic technologies and bottom-up synthesis processes have been used to engineer such wires, critical challenges remain in growing uniform atomic-scale crystalline wires and constructing their network structures. Here we discover a simple method to fabricate atomic-scale wires with various arrangements, including stripes, X-, Y-junctions, and nanorings. Single-crystalline atomic-scale wires of a Mott insulator, whose band gap is comparable to those of wide-gap semiconductors, are spontaneously grown on graphite substrates \DEL{and epitaxial monolayer graphene on SiC }by pulsed-laser deposition. These wires are one-unit-cell-thick and have an exact width of two- and four-unit-cells (1.4 and 2.8\,nm) and lengths up to a few $μm$. We show that the non-equilibrium reaction-diffusion processes may play an essential role in atomic pattern formation. Our findings offer a new perspective on the non-equilibrium self-organization phenomena on an atomic scale, paving a unique way for the quantum architecture of nano-network.

cond-mat.mes-hall↗

arXiv2023

Interacting topological quantum chemistry of Mott atomic limits

Topological quantum chemistry (TQC) is a successful framework for identifying (noninteracting) topological materials. Based on the symmetry eigenvalues of Bloch eigenstates at maximal momenta, which are attainable from first principles calculations, a band structure can either be classified as an atomic limit, in other words adiabatically connected to independent electronic orbitals on the respective crystal lattice, or it is topological. For interacting systems, there is no single-particle band structure and hence, the TQC machinery grinds to a halt. We develop a framework analogous to TQC, but employing $n$-particle Green's function to classify interacting systems. Fundamentally, we define a class of interacting reference states that generalize the notion of atomic limits, which we call Mott atomic limits, and are symmetry protected topological states. Our formalism allows to fully classify these reference states (with $n=2$), which can themselves represent symmetry protected topological states. We present a comprehensive classification of such states in one-dimension and provide numerical results on model systems. With this, we establish Mott atomic limit states as a generalization of the atomic limits to interacting systems.

cond-mat.str-el↗

arXiv2023

Detecting topological phases in the square-octagon lattice with statistical methods

Electronic systems living on Archimedean lattices such as kagome and square-octagon networks are presently being intensively discussed for the possible realization of topological insulating phases. Coining the most interesting electronic topological states in an unbiased way is however not straightforward due to the large parameter space of possible Hamiltonians. A possible approach to tackle this problem is provided by a recently developed statistical learning method [T. Mertz and R. Valentí, Phys. Rev. Research 3, 013132 (2021)], based on the analysis of a large data sets of randomized tight-binding Hamiltonians labeled with a topological index. In this work, we complement this technique by introducing a feature engineering approach which helps identifying polynomial combinations of Hamiltonian parameters that are associated with non-trivial topological states. As a showcase, we employ this method to investigate the possible topological phases that can manifest on the square-octagon lattice, focusing on the case in which the Fermi level of the system lies at a high-order van Hove singularity, in analogy to recent studies of topological phases on the kagome lattice at the van Hove filling.

cond-mat.str-el↗

arXiv2023

Heterogeneous Ta-dichalcogenide bilayer: heavy fermions or doped Mott physics?

Controlling and understanding electron correlations in quantum matter is one of the most challenging tasks in materials engineering. In the past years a plethora of new puzzling correlated states have been found by carefully stacking and twisting two-dimensional van der Waals materials of different kind. Unique to these stacked structures is the emergence of correlated phases not foreseeable from the single layers alone. In Ta-dichalcogenide heterostructures made of a good metallic 1H- and a Mott-insulating 1T-layer, recent reports have evidenced a cross-breed itinerant and localized nature of the electronic excitations, similar to what is typically found in heavy fermion systems. Here, we put forward a new interpretation based on first-principles calculations which indicates a sizeable charge transfer of electrons (0.4-0.6 e) from 1T to 1H layers at an elevated interlayer distance. We accurately quantify the strength of the interlayer hybridization which allows us to unambiguously determine that the system is much closer to a doped Mott insulator than to a heavy fermion scenario. Ta-based heterolayers provide therefore a new ground for quantum-materials engineering in the regime of heavily doped Mott insulators hybridized with metallic states at a van der Waals distance.

cond-mat.str-el↗

arXiv2023

Microscopic analysis of the valence transition in tetragonal EuPd$_2$Si$_2$

Under temperature or pressure tuning, tetragonal EuPd$_2$Si$_2$ is known to undergo a valence transition from nearly divalent to nearly trivalent Eu accompanied by a volume reduction. Albeit intensive work, its microscopic origin is still being discussed. Here, we investigate the mechanism of the valence transition under volume compression by $ab~initio$ density functional theory (DFT) calculations. Our analysis of the electronic and magnetic properties of EuPd$_2$Si$_2$ when approaching the valence transition shows an enhanced $c$-$f$ hybridization between localized Eu 4$f$ states and itinerant conduction states (Eu 5$d$, Pd 4$d$, and Si 3$p$) where an electronic charge redistribution takes place. We observe that the change in the electronic structure is intimately related to the volume reduction where Eu-Pd(Si) bond lengths shorten and, for the transition to happen, we trace the delicate balance between electronic bandwidth, crystal field splitting, Coulomb repulsion, Hund's coupling and spin-orbit coupling. In a next step we compare and benchmark our DFT results to surface-sensitive photoemission data in which the mixed-valent properties of EuPd$_2$Si$_2$ are reflected in a simultaneous observation of divalent and trivalent signals from the Eu $4f$ shell. The study serves as well to explore the limits of density functional theory and the choice of exchange correlation functionals to describe such a phenomenon as a valence transition.

cond-mat.str-el↗

arXiv2022

Hybrid s-wave superconductivity in CrB$_2$

In a metal with multiple Fermi pockets, the formation of s-wave superconductivity can be conventional due to electron-phonon coupling or unconventional due to spin fluctuations. We analyze the hexagonal diboride CrB$_2$, which is an itinerant antiferromagnet at ambient conditions and turns superconducting upon increasing pressure. While the high pressure behavior of T$_c$ suggests conventional s-wave pairing, we find that spin fluctuations promoting unconventional s-wave pairing become important in the vicinity of the antiferromagnetic dome. As the symmetry class of the s-wave state is independent of its underlying mechanism, we argue that CrB$_2$ is a realization of a hybrid s-wave superconductor where unconventional and conventional s-wave mechanisms team up to form a joint superconducting dome.

cond-mat.supr-con↗

arXiv2022

Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3

Layered α-RuCl3 is a promising material to potentially realize the long-sought Kitaev quantum spin liquid with fractionalized excitations. While evidence of this exotic state has been reported under a modest in-plane magnetic field, such behavior is largely inconsistent with theoretical expectations of Kitaev phases emerging only in out-of-plane fields. These predicted field-induced states have been mostly out of reach due to the strong easy-plane anisotropy of bulk crystals, however. We use a combination of tunneling spectroscopy, magnetotransport, electron diffraction, and ab initio calculations to study the layer-dependent magnons, anisotropy, structure, and exchange coupling in atomically thin samples. Due to structural distortions, the sign of the average off-diagonal exchange changes in monolayer α-RuCl3, leading to a reversal of magnetic anisotropy to easy-axis. Our work provides a new avenue to tune the magnetic interactions in α-RuCl3 and allows theoretically predicted quantum spin liquid phases for out-of-plane fields to be more experimentally accessible.

cond-mat.str-el↗

arXiv2022

Superconductivity from repulsive interactions on the kagome lattice

The discovery of superconductivity in layered vanadium-based kagome metals $A$V$_3$Sb$_5$ ($A$: K, Rb, Cs) has added a new family of materials to the growing class of possible unconventional superconductors. However, the nature of the superconducting pairing in these materials remains elusive. We present a microscopic theoretical study of the leading superconducting instabilities on the kagome lattice based on spin- and charge-fluctuation mediated Cooper pairing. The applied methodology includes effects of both on-site and nearest-neighbor repulsive Coulomb interactions. Near the upper van Hove filling -- relevant for the $A$V$_3$Sb$_5$ materials -- we find a rich phase diagram with several pairing symmetries being nearly degenerate. In particular, while a substantial fraction of the phase diagram is occupied by a spin-singlet order parameter transforming as a two-dimensional irreducible representation of the point group, several nodal spin-triplet pairing states remain competitive. We compute the band and interaction parameter-dependence of the hierarchy of the leading superconducting instabilities, and determine the detailed momentum dependence of the resulting preferred gap structures. Crucially, for moderate values of the interaction parameters, the individual pairing states depend strongly on momentum and exhibit multiple nodes on the Fermi surface. We discuss the properties of these superconducting gap structures in light of recent experimental developments of the $A$V$_3$Sb$_5$ materials.

cond-mat.supr-con↗

arXiv2022

Charge-density waves in kagome-lattice extended Hubbard models at the van Hove filling

The Hubbard model on the kagome lattice is presently often considered as a minimal model to describe the rich low-temperature behavior of AV$_{3}$Sb$_{5}$ compounds (with A=K, Rb, Cs), including charge-density waves (CDWs), superconductivity, and possibly broken time-reversal symmetry. Here, we investigate, via variational Jastrow-Slater wave functions, the properties of its ground state when both onsite $U$ and nearest-neighbor $V$ Coulomb repulsions are considered at the van Hove filling. Our calculations reveal the presence of different interaction-driven CDWs and, contrary to previous renormalization-group studies, the absence of ferromagnetism and charge- or spin-bond order. No signatures of chiral phases are detected. Remarkably, the CDWs triggered by the nearest-neighbor repulsion possess charge disproportionations that are not compatible with the ones observed in AV$_{3}$Sb$_{5}$. As an alternative mechanism to stabilize charge-bond order, we consider the electron-phonon interaction, modeled by coupling the hopping amplitudes to quantum phonons, as in the Su-Schrieffer-Heeger model. Our results show the instability towards a tri-hexagonal distortion with $2\times 2$ periodicity, in a closer agreement with experimental findings.

cond-mat.str-el↗

arXiv2022

Ultra-sharp lateral $p\text{-}n$ junctions in modulation-doped graphene

We demonstrate ultra-sharp (${\lesssim}\,10\text{ nm}$) lateral $p\text{-}n$ junctions in graphene using electronic transport, scanning tunneling microscopy, and first principles calculations. The $p\text{-}n$ junction lies at the boundary between differentially-doped regions of a graphene sheet, where one side is intrinsic and the other is charge-doped by proximity to a flake of $α$-RuCl$_3$ across a thin insulating barrier. We extract the $p\text{-}n$ junction contribution to the device resistance to place bounds on the junction width. We achieve an ultra-sharp junction when the boundary between the intrinsic and doped regions is defined by a cleaved crystalline edge of $α$-RuCl$_3$ located 2 nm from the graphene. Scanning tunneling spectroscopy in heterostructures of graphene, hexagonal boron nitride, and $α$-RuCl$_3$ shows potential variations on a sub-10 nm length scale. First principles calculations reveal the charge-doping of graphene decays sharply over just nanometers from the edge of the $α$-RuCl$_3$ flake.

cond-mat.mes-hall↗