arXiv subjects

Alberto Nocera

Publications and source records attributed to Alberto Nocera.

50 records · Page 3Linked to original sources

arXiv2020

Comprehensive study of the phase diagram of the spin-1/2 Kitaev-Heisenberg-Gamma chain

A central question on Kitaev materials is the effects of additional couplings on the Kitaev model which is proposed to be a candidate for realizing topological quantum computations. However, two spatial dimension typically suffers the difficulty of lacking controllable approaches. In this work, using a combination of powerful analytical and numerical methods available in one dimension, we perform a comprehensive study on the phase diagram of a one-dimensional version of the spin-1/2 Kitaev-Heisenberg-Gamma model in its full parameter space. A strikingly rich phase diagram is found with nine distinct phases, including four Luttinger liquid phases, a ferromagnetic phase, a Néel ordered phase, an ordered phase of distorted-spiral spin alignments, and two ordered phase which both break a $D_3$ symmetry albeit in different ways, where $D_3$ is the dihedral group of order six. Our work paves the way for studying one-dimensional Kitaev materials and may provide hints to the physics in higher dimensional situations.

cond-mat.str-el↗

arXiv2020

Spin wave theory of one-dimensional generalized Kitaev model

In this work, we perform a combination of classical and spin wave analysis on the one-dimensional spin-$S$ Kitaev-Heisenberg-Gamma model in the region of an antiferromagnetic Kitaev coupling. Four phases are found, including a Néel ordered phase, a phase with $O_h\rightarrow D_3$ symmetry breaking, and "$D_3$-breaking I, II" phases which both break $D_3$ symmetries albeit in different ways, where $O_h$ is the full octahedral group and $D_3$ is the dihedral group of order six. The lowest-lying spin wave mass is calculated perturbatively in the vicinity of the hidden SU(2) symmetric ferromagnetic point.

cond-mat.str-el↗

arXiv2020

Emergence of Superconductivity in Doped Multiorbital Hubbard Chains

We introduce a variational state for one-dimensional two-orbital Hubbard models that intuitively explains the recent computational discovery of pairing in these systems when hole doped. Our Ansatz is an optimized linear superposition of Affleck-Kennedy-Lieb-Tasaki valence bond states, rendering the combination a valence bond liquid dubbed Orbital Resonant Valence Bond. We show that the undoped (one electron/orbital) quantum state of two sites coupled into a global spin singlet is exactly written employing only spin-1/2 singlets linking orbitals at nearest-neighbor sites. Generalizing to longer chains defines our variational state visualized geometrically expressing our chain as a two-leg ladder, with one orbital per leg. As in Anderson's resonating valence-bond state, our undoped variational state contains preformed singlet pairs that via doping become mobile leading to superconductivity. Doped real materials with one-dimensional substructures, two near-degenerate orbitals, and intermediate Hubbard U/W strengths -- W the carrier's bandwidth -- could realize spin-singlet pairing if on-site anisotropies are small. If these anisotropies are robust, spin-triplet pairing emerges.

cond-mat.str-el↗

arXiv2020

Spectroscopic signatures of next-nearest-neighbor hopping in the charge and spin dynamics of doped one-dimensional antiferromagnets

We study the impact of next-nearest-neighbor (nnn) hopping on the low-energy collective excitations of strongly correlated doped antiferromagnetic cuprate spin chains. Specifically, we use exact diagonalization and the density matrix renormalization group method to study the single-particle spectral function, the dynamical spin and charge structure factors, and the Cu $L$-edge resonant inelastic x-ray scattering (RIXS) intensity of the doped $t$-$t^\prime$-$J$ model for a set of $t^\prime$ values. We find evidence for the breakdown of spin-charge separation as $|t^\prime|$ increases and identify its fingerprints in the dynamical response functions. The inclusion of nnn hopping couples the spinon and holon excitations, resulting in the formation of a spin-polaron, where a ferromagnetic spin polarization cloud dresses the doped carrier. The spin-polaron manifests itself as additional spectral weight in the dynamical correlation functions, which appear simultaneously in the spin- and charge-sensitive channels. We also demonstrate that RIXS can provide a unique view of the spin-polaron, due to its sensitivity to both the spin and charge degrees of freedom.

cond-mat.str-el↗

arXiv2020

Phase diagram of the spin-1/2 Kitaev-Gamma chain and emergent SU(2) symmetry

We study the phase diagram of a one-dimensional version of the Kitaev spin-1/2 model with an extra ``$Γ$-term", using analytical, density matrix renormalization group and exact diagonalization methods. Two intriguing phases are found. In the gapless phase, although the exact symmetry group of the system is discrete, the low energy theory is described by an emergent SU(2)$_1$ Wess-Zumino-Witten (WZW) model. On the other hand, the spin-spin correlation functions exhibit SU(2) breaking prefactors, even though the exponents and the logarithmic corrections are consistent with the SU(2)$_1$ predictions. A modified nonabelian bosonization formula is proposed to capture such exotic emergent ``partial" SU(2) symmetry. In the ordered phase, there is numerical evidence for an $O_h\rightarrow D_4$ spontaneous symmetry breaking.

cond-mat.str-el↗

arXiv2020

BCS-BEC crossover in a $(t_{2g})^4$ Excitonic Magnet

The condensation of spin-orbit-induced excitons in $(t_{2g})^4$ electronic systems is attracting considerable attention. In the large Hubbard U limit, antiferromagnetism was proposed to emerge from the Bose-Einstein Condensation (BEC) of triplons ($J_{\textrm{eff}} = 1$). In this publication, we show that even for the weak and intermediate U regimes, the spin-orbit exciton condensation is possible leading also to staggered magnetic order. The canonical electron-hole excitations (excitons) transform into local triplon excitations at large U , and this BEC strong coupling regime is smoothly connected to the intermediate U excitonic insulator region. We solved the degenerate three-orbital Hubbard model with spin-orbit coupling ($λ$) in one-dimensional geometry using the Density Matrix Renormalization Group, while in two-dimensional square clusters we use the Hartree-Fock approximation (HFA). Employing these techniques, we provide the full $λ$ vs U phase diagrams for both one- and two- dimensional lattices. Our main result is that at the intermediate Hubbard U region of our focus, increasing $λ$ at fixed U the system transitions from an incommensurate spin-density-wave metal to a Bardeen-Cooper-Schrieffer (BCS) excitonic insulator, with coherence length r coh of O(a) and O(10a) in 1d and 2d, respectively, with a the lattice spacing. Further increasing $λ$, the system eventually crosses over to the BEC limit (with r coh << a).

cond-mat.str-el↗

arXiv2019

Theoretical study of the spin and charge dynamics of two-leg ladders as probed by resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) has become an important tool for studying elementary excitations in correlated materials. Here, we present a systematic theoretical investigation of the Cu L-edge RIXS spectra of undoped and doped cuprate two-leg spin-ladders in both the non-spin-conserving (NSC) and spin-conserving (SC) channels. The spectra are rich and host many exotic excitations. In the NSC-channel of the undoped case, we identify one-triplon and bound $\textit{triplet}$ two-triplon excitations in the strong-rung coupling limit, as well as confined spinons in the weak-rung coupling limit. In the doped case, we observe a quasiparticle excitation formed from a bound charge and spin-$\frac{1}{2}$ in the strong-rung coupling limit. In the SC-channel, we also identify several new features, including bound $\textit{singlet}$ two-triplon excitations and confined spinons in the undoped ladders in the strong- and weak-rung coupling limits, respectively. Conversely, in the doped case, the SC channel primarily probes both gapless and gapped charge excitations. Finally, we revisit the available data for the ladder compound Sr$_{14}$Cu$_{24}$O$_{41}$ in the context of our results.

cond-mat.str-el↗

arXiv2019

Novel Block Excitonic Condensate at $n=3.5$ in a Spin-Orbit Coupled $t_{2g}$ Multiorbital Hubbard Model

Theoretical studies recently predicted the condensation of spin-orbit excitons at momentum $q$=$π$ in $t_{2g}^4$ spin-orbit coupled three-orbital Hubbard models at electronic density $n=4$. In parallel, experiments involving iridates with non-integer valence states for the Ir ions are starting to attract considerable attention. In this publication, using the density matrix renormalization group technique we present evidence for the existence of a novel excitonic condensate at $n=3.5$ in a one-dimensional Hubbard model with a degenerate $t_{2g}$ sector, when in the presence of spin-orbit coupling. At intermediate Hubbard $U$ and spin-orbit $λ$ couplings, we found an excitonic condensate at the unexpected momentum $q$=$π/2$ involving $j_{\textrm{eff}}=3/2,m=\pm1/2$ and $j_{\textrm{eff}}=1/2,m=\pm1/2$ bands in the triplet channel, coexisting with an also unexpected block magnetic order. We also present the entire $λ$ vs $U$ phase diagram, at a fixed and robust Hund coupling. Interestingly, this new `block excitonic phase' is present even at large values of $λ$, unlike the $n=4$ excitonic phase discussed before. Our computational study helps to understand and predict the possible magnetic phases of materials with $d^{3.5}$ valence and robust spin-orbit coupling.

cond-mat.str-el↗

arXiv2018

Fingerprints of an Exotic Orbital-Selective Mott Phase in the Block Magnetic State of BaFe$_2$Se$_3$ Ladders

Resonant Inelastic X-Ray Scattering (RIXS) experiments on the iron-based ladder BaFe$_2$Se$_3$ unveiled an unexpected two-peak structure associated with local orbital ($dd$) excitations in a block-type antiferromagnetic phase. A mixed character between correlated band-like and localized excitations was also reported. Here, we use the density matrix renormalization group method to calculate the momentum-resolved charge- and orbital-dynamical response functions of a multi-orbital Hubbard chain. Remarkably, our results qualitatively resemble the BaFe$_2$Se$_3$ RIXS data, while also capturing the presence of long-range magnetic order as found in neutron scattering, $only$ when the model is in an exotic orbital-selective Mott phase (OSMP). In the calculations, the experimentally observed zero-momentum transfer RIXS peaks correspond to excitations between itinerant and Mott insulating orbitals. We provide experimentally testable predictions for the momentum-resolved charge and orbital dynamical structures, which can provide further insight into the OSMP regime of BaFe$_2$Se$_3$.

cond-mat.str-el↗

arXiv2018

Multi-spinon and holon excitations probed by resonant inelastic x-ray scattering on doped one-dimensional antiferromagnets

Resonant inelastic x-ray scattering (RIXS) at the oxygen $K$-edge has recently accessed multi-spinon excitations in the one-dimensional antiferromagnet (1D-AFM) \sco, where four-spinon excitations are resolved separately from the two-spinon continuum. This technique, therefore, provides new opportunities to study fractionalized quasiparticle excitations in doped 1D-AFMs. To this end, we carried out exact diagonalization studies of the doped $t$-$J$ model and provided predictions for oxygen $K$-edge RIXS experiments on doped 1D-AFMs. We show that the RIXS spectra are rich, containing distinct two- and four-spinon excitations, dispersive (anti)holon excitations, and combinations thereof. Our results highlight how RIXS complements inelastic neutron scattering experiments by accessing additional charge and spin components of fractionalized quasiparticles.

cond-mat.str-el↗

arXiv2017

Finite temperature dynamics of the Mott insulating Hubbard chain

We study the dynamical response of the half-filled one-dimensional(1d) Hubbard model for a range of interaction strengths $U$ and temperatures $T$ by a combination of numerical and analytical techniques. Using time-dependent density matrix renormalization group (tDMRG) computations we find that the single-particle spectral function undergoes a crossover to a spin-incoherent Luttinger liquid regime at temperatures $T \sim J=4t^2/U$ for sufficiently large $U > 4t$. At smaller values of $U$ and elevated temperatures the spectral function is found to exhibit two thermally broadened bands of excitations, reminiscent of what is found in the Hubbard-I approximation. The dynamical density-density response function is shown to exhibit a finite temperature resonance at low frequencies inside the Mott gap, with a physical origin similar to the Villain mode in gapped quantum spin chains. We complement our numerical computations by developing an analytic strong-coupling approach to the low-temperature dynamics in the spin-incoherent regime.

cond-mat.str-el↗

arXiv2017

Density matrix renormalization group study of a three-orbital Hubbard model with spin-orbit coupling in one dimension

Using the Density Matrix Renormalization Group technique we study the effect of spin-orbit coupling on a three-orbital Hubbard model in the $(t_{2g})^{4}$ sector and in one dimension. Fixing the Hund coupling to a robust value compatible with some multiorbital materials, we present the phase diagram varying the Hubbard $U$ and spin-orbit coupling $λ$, at zero temperature. Our results are shown to be qualitatively similar to those recently reported using the Dynamical Mean Field Theory in higher dimensions, providing a robust basis to approximate many-body techniques. Among many results, we observe an interesting transition from an orbital-selective Mott phase to an excitonic insulator with increasing $λ$ at intermediate $U$. In the strong $U$ coupling limit, we find a non-magnetic insulator with an effective angular momentum $\langle(\textbf{J}^{eff})^{2}\rangle \ne 0$ near the excitonic phase, smoothly connected to the $\langle(\textbf{J}^{eff})^{2}\rangle = 0$ regime. We also provide a list of quasi-one dimensional materials where the physics discussed in this publication could be realized.

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arXiv2016

Unconventional fermionic pairing states in a monochromatically tilted optical lattice

We study the one-dimensional attractive Fermionic Hubbard model under the influence of periodic driving with the time-dependent density matrix renormalization group method. We show that the system can be driven into an unconventional pairing state characterized by a condensate made of Cooper-pairs with a finite center-of-mass momentum similar to a Fulde-Ferrell state. We obtain results both in the laboratory and the rotating reference frames demonstrating that the momentum of the condensate can be finely tuned by changing the ratio between the amplitude and the frequency of the driving. In particular, by quenching this ratio to the value corresponding to suppression of the tunnelling and the Coulomb interaction strength to zero, we are able to "freeze" the condensate. We finally study the effects of different initial conditions, and compare our numerical results to those obtained from a time-independent Floquet theory in the large frequency regime. Our work offers the possibility of engineering and controlling unconventional pairing states in fermionic condensates.

cond-mat.quant-gas↗

arXiv2016

Magnetic properties and pairing tendencies of the iron-based superconducting ladder BaFe$_2$S$_3$: combined ab initio and density matrix renormalization group study

The recent discovery of superconductivity under high pressure in the two-leg ladder compound BaFe$_2$S$_3$ [H. Takahashi et al., Nature Materials 14, 1008 (2015)] opens a broad avenue of research, because it represents the first report of pairing tendencies in a quasi one-dimensional iron-based high critical temperature superconductor. Similarly as in the case of the cuprates, ladders and chains can be far more accurately studied using many-body techniques and model Hamiltonians than their layered counterparts, particularly if several orbitals are active. In this publication, we derive a two-orbital Hubbard model from first principles that describes individual ladders of BaFe$_2$S$_3$. The model is studied with the density matrix renormalization group. These first reported results are exciting for two reasons: (i) at half-filling, ferromagnetic order emerges as the dominant magnetic pattern along the rungs of the ladder, and antiferromagnetic order along the legs, in excellent agreement with neutron experiments; (ii) pairs form in the strong coupling regime, as found by studying the binding energy of two holes doped on the half-filled system. In addition, Orbital Selective Mott Phase characteristics develop with doping, with only one Wannier orbital receiving the hole carriers while the other remains half-filled. These results suggest that the analysis of models for iron-based two-leg ladders could clarify the origin of pairing tendencies and other exotic properties of iron-based high critical temperature superconductors.

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