arXiv subjects

Elbio Dagotto

Publications and source records attributed to Elbio Dagotto.

At least 127 records · Page 7Linked to original sources

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.

cond-mat.str-el↗

arXiv2016

Orbital-selective Mott phases of a one-dimensional three-orbital Hubbard model studied using computational techniques

A recently introduced one-dimensional three-orbital Hubbard model displays orbital-selective Mott phases with exotic spin arrangements such as spin block states [J. Rincón {\em et al.}, Phys. Rev. Lett. \textbf{112}, 106405 (2014)]. In this publication we show that the Constrained-Path Quantum Monte Carlo (CPQMC) technique can accurately reproduce the phase diagram of this multiorbital one-dimensional model, paving the way to future CPQMC studies in systems with more challenging geometries, such as ladders and planes. The success of this approach relies on using the Hartree-Fock technique to prepare the trial states needed in CPQMC. We also study a simplified version of the model where the pair-hopping term is neglected and the Hund coupling is restricted to its Ising component. The corresponding phase diagrams are shown to be only mildly affected by the absence of these technically difficult-to-implement terms. This is confirmed by additional Density Matrix Renormalization Group and Determinant Quantum Monte Carlo calculations carried out for the same simplified model, with the latter displaying only mild Fermion sign problems. We conclude that these methods are able to capture quantitatively the rich physics of the several orbital-selective Mott phases (OSMP) displayed by this model, thus enabling computational studies of the OSMP regime in higher dimensions, beyond static or dynamic mean field approximations.

cond-mat.str-el↗

arXiv2016

Inversion of ferrimagnetic magnetization by ferroelectric switching via a novel magnetoelectric coupling

Although several multiferroic materials/heterostructures have been extensively studied, finding strong magnetoelectric couplings for the electric field control of the magnetization remains challenging. Here, a novel interfacial magnetoelectric coupling based on three components (ferroelectric dipole, magnetic moment, and antiferromagnetic order) is analytically formulated. As an extension of carrier-mediated magnetoelectricity, the new coupling is shown to induce an electric-magnetic hysteresis loop. Realizations employing BiFeO$_3$ bilayers grown along the [$111$] axis are proposed. Without involving magnetic phase transitions, the magnetization orientation can be switched by the carrier modulation driven by the field effect, as confirmed using first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2016

Bicollinear Antiferromagnetic Order, Monoclinic Distortion, and Reversed Resistivity Anisotropy in FeTe as a Result of Spin-Lattice Coupling

The bicollinear antiferromagnetic order experimentally observed in FeTe is shown to be stabilized by the coupling $\tilde g_{12}$ between monoclinic lattice distortions and the spin-nematic order parameter with $B_{\rm 2g}$ symmetry, within a three-orbital spin-fermion model studied with Monte Carlo techniques. A finite but small value of $\tilde g_{12}$ is required, with a concomitant lattice distortion compatible with experiments, and a tetragonal-monoclinic transition strongly first order. Remarkably, the bicollinear state found here displays a planar resistivity with the "reversed" puzzling anisotropy discovered in transport experiments.Orthorhombic distortions are also incorporated and phase diagrams interpolating between pnictides and chalcogenides are presented. We conclude that the spin-lattice coupling discussed here is sufficient to explain the challenging properties of FeTe.

cond-mat.str-el↗

arXiv2016

Orbital Selective Directional Conductor in the Two Orbital Hubbard Model

Employing a recently developed many-body technique that allows for the incorporation of thermal effects, the rich phase diagram of a two dimensional two orbital (degenerate $d_{xz}$ and $d_{yz}$) Hubbard model is presented varying temperature and the repulsion $U$. Our main result is the finding at intermediate $U$ of a novel antiferromagnetic orbital selective state where an effective dimensional reduction renders one direction insulating and the other metallic. Possible realizations of this state are discussed. In addition we also study nematicity above the Néel temperature. After a careful finite-size scaling analysis the nematicity temperature window appears to survive in the bulk limit although it is very narrow.

cond-mat.str-el↗

arXiv2016

On-site Atractive Multiorbital Hamiltonian for $d$-Wave Superconductors

We introduce a two-orbital Hamiltonian on a square lattice that contains on-site attractive interactions involving the two $e_g$ orbitals. Via a canonical mean-field procedure similar to the one applied to the well-known negative-$U$ Hubbard model, it is shown that the new model develops $d$-wave ($B_{1g}$) superconductivity with nodes along the diagonal directions of the square Brillouin zone. This result is also supported by exact diagonalization of the model in a small cluster. The expectation is that this relatively simple attractive model could be used to address the properties of multiorbital $d$-wave superconductors in the same manner that the negative-$U$ Hubbard model is widely applied to the study of the properties of $s$-wave single-orbital superconductors. In particular, we show that by splitting the $e_g$ orbitals and working at three-quarters filling, such that the $x^2-y^2$ orbital dominates at the Fermi level but the $3z^2-r^2$ orbital contribution is nonzero, the $d$-wave pairing state found here phenomenologically reproduces several properties of the superconducting state of the high $T_c$ cuprates.

cond-mat.supr-con↗

arXiv2015

Strain Doping: Reversible Single-Axis Control of a Complex Oxide Lattice via Helium Implantation

We report on the use of helium ion implantation to independently control the out-of-plane lattice constant in epitaxial La0.7Sr0.3MnO3 thin films without changing the in-plane lattice constants. The process is reversible by a vacuum anneal. Resistance and magnetization measurements show that even a small increase in the out-of-plane lattice constant of less than 1% can shift the metal-insulator transition and Curie temperatures by more than 100 °C. Unlike conventional epitaxy-based strain tuning methods which are constrained not only by the Poisson effect but by the limited set of available substrates, the present study shows that strain can be independently and continuously controlled along a single axis. This permits novel control over orbital populations through Jahn-Teller effects, as shown by Monte Carlo simulations on a double-exchange model. The ability to reversibly control a single lattice parameter substantially broadens the phase space for experimental exploration of predictive models and leads to new possibilities for control over materials' functional properties.

cond-mat.str-el↗

arXiv2015

Isotropic Quenched Disorder Triggers a Robust Nematic State in Electron-Doped Pnictides

The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic quenched disorder strength by means of computational techniques applied to a three-orbital ($xz$, $yz$, $xy$) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. In this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, our Monte Carlo simulations reveal that the fast reduction with doping of the Néel $T_N$ and the structural $T_S$ transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi Surface produced by electron doping affect only slightly both critical temperatures.

cond-mat.supr-con↗

arXiv2015

Parallelized Traveling Cluster Approximation to Study Numerically Spin-Fermion Models on Large Lattices

Lattice spin-fermion models are important to study correlated systems where quantum dynamics allows for a separation between slow and fast degrees of freedom. The fast degrees of freedom are treated quantum mechanically while the slow variables, generically refereed to as the "spins", are treated classically. At present, exact diagonalization coupled with classical Monte Carlo (ED+MC) is extensively used to solve numerically a general class of lattice spin-fermion problems. In this common setup, the classical variables (spins) are treated via the standard MC method while the fermion problem is solved by exact diagonalization. The "Traveling Cluster Approximation" (TCA) is a real space variant of the ED+MC method that allows to solve spin-fermion problems on lattice sizes with up to $10^3$ sites. In this publication, we present a novel reorganization of the TCA algorithm in a manner that can be efficiently parallelized. This allows us to solve generic spin-fermion models easily on $10^4$ lattice sites and with some effort on $10^5$ lattice sites, representing the record lattice sizes studied for this family of models.

cond-mat.str-el↗

arXiv2014

Quantum phase transition between orbital-selective Mott states in Hund's metals

We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard $U$, at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically-driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.

cond-mat.str-el↗

arXiv2014

Testing the Monte Carlo - Mean Field approximation in the one-band Hubbard model

The canonical one-band Hubbard model is studied using a computational method that mixes the Monte Carlo procedure with the mean field approximation. This technique allows us to incorporate thermal fluctuations and the development of short-range magnetic order above ordering temperatures, contrary to the crude finite-temperature Hartree-Fock approximation, which incorrectly predicts a Néel temperature $T_N$ that grows linearly with the Hubbard $U/t$. The effective model studied here contains quantum and classical degrees of freedom. It thus belongs to the "spin fermion" model family widely employed in other contexts. Using exact diagonalization, supplemented by the traveling cluster approximation, for the fermionic sector, and classical Monte Carlo for the classical fields, the Hubbard $U/t$ vs. temperature $T/t$ phase diagram is studied employing large three and two dimensional clusters. We demonstrate that the method is capable of capturing the formation of local moments in the normal state without long-range order, the non-monotonicity of $T_N$ with increasing $U/t$, the development of gaps and pseudogaps in the density of states, and the two-peak structure in the specific heat. Extensive comparisons with determinant quantum Monte Carlo results suggest that the present approach is qualitatively, and often quantitatively, accurate, particularly at intermediate and high temperatures. Finally, we study the Hubbard model including plaquette diagonal hopping (i.e. the $t-t^\prime$ Hubbard model) in two dimensions and show that our approach allows us to study low temperature properties where determinant quantum Monte Carlo fails due to the fermion sign problem. Future applications of this method include multi-orbital Hubbard models such as those needed for iron-based superconductors.

cond-mat.str-el↗

arXiv2014

Photoexcitation of electronic instabilities in one-dimensional charge-transfer systems

We investigate the real-time dynamics of photoexcited electronic instabilities in a charge-transfer system model, using the time-dependent density matrix renormalization group method. The model of choice was the quarter-filled one-dimensional extended Peierls-Hubbard Hamiltonian interacting with classical few-cycle electromagnetic radiation. The results show that only one electronic instability drives the main features of the photogenerated time-dependent behavior. Indeed, the photoresponse of the system shows a large enhancement of the $4k_F$ (bond and charge) instability whereas the $2k_F$ state remains largely unaffected. This conclusion holds regardless of the nature of the optical excitations and whether the system is perturbed resonantly or not. Our results suggest potential applications of charge-transfer systems with slow phononic dynamics as optoelectronic switching devices.

cond-mat.str-el↗

arXiv2014

BaFe$_2$Se$_3$: a high $T_{\rm C}$ magnetic multiferroic with large ferrielectric polarization

The iron-selenides are important because of their superconducting properties. Here, an unexpected phenomenon is predicted to occur in an iron-selenide compound with a quasi-one-dimensional ladder geometry: BaFe$_2$Se$_3$ should be a magnetic ferrielectric system, driven by its magnetic block order via exchange striction. A robust performance (high $T_{\rm C}$ and large polarization) is expected. Different from most multiferroics, BaFe$_2$Se$_3$ is ferrielectric, with a polarization that mostly cancels between ladders. However, its strong magnetostriction still produces a net polarization that is large ($\sim$$0.1$ $μ$C/cm$^2$) as compared with most magnetic multiferroics. Its fully ferroelectric state, with energy only slightly higher than the ferrielectric, has a giant improper polarization $\sim$$2-3$ $μ$C/cm$^2$.

cond-mat.mtrl-sci↗

arXiv2014

Orbital occupancy and charge doping in iron-based superconductors

Iron-based superconductors (FBS) comprise several families of compounds having the same atomic building blocks for superconductivity, but large discrepancies among their physical properties. A longstanding goal in the field has been to decipher the key underlying factors controlling TC and the various doping mechanisms. In FBS materials this is complicated immensely by the different crystal and magnetic structures exhibited by the different families. In this paper, using aberration-corrected scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), we observe a universal behavior in the hole concentration and magnetic moment across different families. All the parent materials have the same total number of electrons in the Fe 3d bands; however, the local Fe magnetic moment varies due to different orbital occupancy. Although the common understanding has been that both long-range and local magnetic moments decrease with doping, we find that, near the onset of superconductivity, the local magnetic moment increases and shows a dome-like maximum near optimal doping, where no ordered magnetic moment is present. In addition, we address a longstanding debate concerning how Co substitutions induces superconductivity in the 122 arsenide family, showing that the 3d band filling increases a function of doping. These new microscopic insights into the properties of FBS demonstrate the importance of spin fluctuations for the superconducting state, reveal changes in orbital occupancy among different families of FBS, and confirm charge doping as one of the mechanisms responsible for superconductivity in 122 arsenides.

cond-mat.supr-con↗

arXiv2014

Diverging Nematic Susceptibility, Physical Meaning of T* scale, and Pseudogap in the Spin Fermion Model for Pnictides

Using Monte Carlo simulations with a tunable uniaxial strain, for the first time the nematic susceptibility of a model for the pnictides is calculated. The results are in good agreement with the experiments by J-H. Chu {\it et al.}, Science {\bf 337}, 710 (2012). Via a Ginzburg-Landau analysis, our study suggests a nematicity in pnictides primarily originating on magnetism, but with the lattice/orbital boosting up critical temperatures and separating the structural $T_S$ and Néel $T_N$ transitions. At $T>T_S$, Curie-Weiss behavior is observed with the characteristic temperature $T^*$ unveiled by Chu {\it et al.} being the $T_N$ of the purely electronic system. In this temperature regime, short-range magnetic order with wavevectors $(π,0)-(0,π)$ induce local nematic fluctuations and a density-of-states pseudogap, compatible with several experiments.

cond-mat.supr-con↗

arXiv2014

Ab initio quantum Monte Carlo calculations of spin superexchange in cuprates: the benchmarking case of Ca$_2$CuO$_3$

In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively excellent agreement with experiment. The variational nature of the QMC total energy allows us to identify the best trial wave function out of the available pool of wave functions, which makes the approach essentially free from adjustable parameters and thus truly ab initio. The present results on magnetic interactions suggest that QMC is capable of accurately describing ground state properties of strongly correlated materials.

cond-mat.str-el↗

arXiv2014

Competition between covalent bonding and charge transfer at complex-oxide interfaces

Here we study the electronic properties of cuprate/manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high $T_c$ superconductor YBa$_2$Cu$_3$O$_{7-δ}$ and the colossal magnetoresistance compound (La,Ca)MnO$_3$. A net transfer of electrons from manganite to cuprate with a peculiar non-monotonic charge profile is observed. Model calculations rationalize the profile in terms of the competition between standard charge transfer tendencies (due to band mismatch), strong chemical bonding effects across the interface, and Cu substitution into the Mn lattice, with different characteristic length scales.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic states of the five-orbital Hubbard model for one-dimensional iron-based superconductors

The magnetic phase diagrams of models for quasi one-dimensional compounds belonging to the iron-based superconductors family are presented. The five-orbital Hubbard model and the real-space Hartree-Fock approximation are employed, supplemented by density functional theory to obtain the hopping amplitudes. Phase diagrams are constructed varying the Hubbard $U$ and Hund $J$ couplings and at zero temperature. The study is carried out at electronic density (electrons per iron) $n = 5.0$, which is of relevance for the already known material TlFeSe$_2$, and also at $n = 6.0$, where representative compounds still need to be synthesized. At $n = 5.0$ there is a clear dominance of staggered spin order along the chain direction. At $n = 6.0$ and the realistic Hund coupling $J/U = 0.25$, the phase diagram is far richer including a variety of ``block'' states involving ferromagnetic clusters that are antiferromagnetically coupled, in qualitative agreement with recent Density Matrix Renormalization Group calculations for the three-orbital Hubbard model in a different context. These block states arise from the competition between ferromagnetic order (induced by double exchange, and prevailing at large $J/U$) and antiferromagnetic order (dominating at small $J/U$). The density of states and orbital compositions of the many phases are also provided.

cond-mat.supr-con↗