Paula Mellado - Academia.edu (original) (raw)

Papers by Paula Mellado

Journal of Physics: Condensed Matter, Feb 24, 2023

We study solitons in a zig-zag lattice of magnetic dipoles. The lattice comprises two sublattices... more We study solitons in a zig-zag lattice of magnetic dipoles. The lattice comprises two sublattices of parallel chains with magnetic dipoles at their vertices. Due to orthogonal easy planes of rotation for dipoles belonging to different sublattices, the total dipolar energy of this system is separable into a sum of symmetric and chiral long-ranged interactions between the magnets where the last takes the form of Dzyaloshinskii–Moriya (DM) coupling. For a specific range of values of the offset between sublattices, the dipoles realize an equilibrium magnetic state in the lattice plane, consisting of one chain settled in an antiferromagnetic (AF) parallel configuration and the other in a collinear ferromagnetic fashion. If the offset grows beyond this value, the internal DM field stabilizes two Bloch domain walls at the edges of the AF chain. The dynamics of these solitons is studied by deriving the long-wavelength lagrangian density for the easy axis antiferromagnet. We find that the chiral couplings between sublattices give rise to an effective magnetic field that stabilizes the solitons in the antiferromagnet. When the chains displace respect to each other, an emergent Lorentz force accelerates the domain walls along the lattice.

ABSTRACT Spin ice in magnetic pyrochlore oxides is a peculiar magnetic state. Like ordinary water... more ABSTRACT Spin ice in magnetic pyrochlore oxides is a peculiar magnetic state. Like ordinary water ice, these materials are in apparent violation with the third law of thermodynamics, which dictates that the entropy of a system in thermal equilibrium vanishes as its temperature approaches absolute zero. In ice, a "zero-point" entropy is retained down to low temperatures thanks to a high number of low-energy positions of hydrogen ions associated with the Bernal-Fowler ice-rules. Spins in pyrochlore oxides Ho2Ti 2O7 and Dy2Ti2O7 exhibit a similar degeneracy of ground states and thus also have a sizable zero-point entropy. A recent discovery of excitations carrying magnetic charges in pyrochlore spin ice adds another interesting dimension to these magnets. This thesis is devoted to a theoretical study of a two-dimensional version of spin ice whose spins reside on kagome, a lattice of corner-sharing triangles. It covers two aspects of this frustrated classical spin system: the dynamics of artificial spin ice in a network of magnetic nanowires and the thermodynamics of crystalline spin ice. Magnetization dynamics in artificial spin ice is mediated by the emission, propagation and absorption of domain walls in magnetic nanowires. The dynamics shows signs of self-organized behavior such as avalanches. The theoretical model compares favorably to recent experiments. The thermodynamics of the microscopic version of spin ice on kagome is examined through analytical calculations and numerical simulations. The results show that, in addition to the high-temperature paramagnetic phase and the low-temperature phase with magnetic order, spin ice on kagome may have an intermediate phase with fluctuating spins and ordered magnetic charges. This work is concluded with a calculation of the entropy of kagome spin ice at zero temperature when one of the sublattices is pinned by an applied magnetic field and the system breaks up into independent spin chains, a case of dimensional reduction.

Bulletin of the American Physical Society, Mar 8, 2019

We studied the real space structure of states in twisted bilayer graphene at the 'magic angle' θ ... more We studied the real space structure of states in twisted bilayer graphene at the 'magic angle' θ = 1.08 •. The flat bands close to charge neutrality are composed of a mix of 'ring' and 'center' orbitals around the AA stacking region. An effective model with localized orbitals is constructed, which necessarily includes more than just the four flat bands. Long-range Coulomb interaction causes a charge-transfer at half-filling of the flat bands from the 'center' to the 'ring' orbitals. Consequently, the Mott phase is a featureless spin-singlet paramagnet. We estimate the effective Heisenberg coupling that favors the singlet coupling to be J = 3.3 K, consistent with experimental values. The superconducting state depends on the nature of the dopants: hole-doping yields p + ipwave whereas electron-doping yields d + id-wave pairing symmetry.

Bulletin of the American Physical Society, Mar 4, 2015

Low-energy states of quantum spin liquids are thought to involve partons living in a gauge-field ... more Low-energy states of quantum spin liquids are thought to involve partons living in a gauge-field background. We study the spectrum of Majorana fermions of Kitaev's honeycomb model on spherical clusters. The gauge field endows the partons with half-integer orbital angular momenta. As a consequence, the multiplicities reflect not the point-group symmetries of the cluster, but rather its projective symmetries, operations combining physical and gauge transformations. The projective symmetry group of the ground state is the double cover of the point group.

Bulletin of the American Physical Society, Mar 3, 2020

Doping twisted bilayer graphene away from charge neutrality leads to an enormous buildup of charg... more Doping twisted bilayer graphene away from charge neutrality leads to an enormous buildup of charge inhomogeneities within each Moiré unit cell. Here we show, using unbiased real-space selfconsistent Hartree calculations on a relaxed lattice, that Coulomb interactions smoothen this charge imbalance by changing the occupation of earlier identified 'ring' orbitals in the AB/BA region and 'center' orbitals at the AA region. For hole doping, this implies an increase of the energy of the states at the Γ point, leading to a further flattening of the flat bands and a pinning of the Van Hove singularity at the Fermi level. The charge smoothening will affect the subtle competition between different possible correlated phases.

Bulletin of the American Physical Society, Mar 17, 2016

Submitted for the MAR16 Meeting of The American Physical Society Unveiling magnetic Hysteresis PA... more Submitted for the MAR16 Meeting of The American Physical Society Unveiling magnetic Hysteresis PAULA MELLADO, ANDRES CON-CHA, DAVID AGUAYO, Adolfo Ibez University-Hysteresis manifests as the lack of retraceability of the magnetization curve in magnetic systems. It has been associated with rotation of magnetization and changes of magnetic domains. However, up to date there has been no realization that allows to separate these coupled mechanisms. We introduce a minimal magnetic system where hysteresis is realized in a simple and minimal fashion. The basic units are a few U(1) ferromagnetic altitudinal rotors placed along a one dimensional chain. They exhibit a dissipative dynamics, interacting via magnetic coupling among them and via Zeeman interaction with the external magnetic field. The system displays a hysteretic behavior starting with N=2 rotors which remains qualitatively invariant as more magnets are added to the chain. We explain this irreversibility by using a model that includes Coulombic interactions between magnetic charges located at the ends of the magnets, zeeman coupling and viscous dissipation. We show that interactions between the unit components is the key element responsible for hysteresis and find that the ability to perceive hysteresis, depends on how the time frequencies of damping and interactions inherent to the system compare with the time frequency set by the external field ramping rate.

arXiv (Cornell University), Oct 27, 2022

We study solitons in a zigzag lattice of magnetic dipoles. The lattice comprises two sublattices ... more We study solitons in a zigzag lattice of magnetic dipoles. The lattice comprises two sublattices of parallel chains with magnetic dipoles at their vertices. Due to orthogonal easy planes of rotation for dipoles belonging to different sublattices, the total dipolar energy of this system is separable into a sum of symmetric and chiral long-ranged interactions between the magnets where the last takes the form of Dzyaloshinskii-Moriya coupling. For a specific range of values of the offset between sublattices, the dipoles realize an equilibrium magnetic state in the lattice plane, consisting of one chain settled in an antiferromagnetic parallel configuration and the other in a collinear ferromagnetic fashion. If the offset grows beyond this value, the internal Dzyaloshinskii-Moriya field stabilizes two Bloch domain walls at the edges of the antiferromagnetic chain. The dynamics of these solitons is studied by deriving the long-wavelength lagrangian density for the easy axis antiferromagnet. We find that the chiral couplings between sublattices give rise to an effective magnetic field that stabilizes the solitons in the antiferromagnet. When the chains displace respect to each other, an emergent Lorentz force accelerates the domain walls along the lattice.

Scientific Reports, Jan 23, 2023

Scientific Reports, Feb 15, 2023

The original Article has been corrected.

Scientific Reports, Jan 26, 2022

arXiv (Cornell University), Jun 25, 2009

Spin ice, a peculiar thermal state of a frustrated ferromagnet on the pyrochlore lattice, has a f... more Spin ice, a peculiar thermal state of a frustrated ferromagnet on the pyrochlore lattice, has a finite entropy density and excitations carrying magnetic charge. By combining analytical arguments and Monte Carlo simulations, we show that spin ice on the two-dimensional kagome lattice orders in two stages. The intermediate phase has ordered magnetic charges and is separated from the paramagnetic phase by an Ising transition. The transition to the low-temperature phase is of the three-state Potts or Kosterlitz-Thouless type, depending on the presence of defects in charge order.

arXiv (Cornell University), Jun 26, 2013

Emergent quasiparticles that arise from the fractionalization of the microscopic degrees of freed... more Emergent quasiparticles that arise from the fractionalization of the microscopic degrees of freedom have been one of the central themes in modern condensed matter physics. The notion of magnetic monopoles, freely moving quasiparticles fragmented from local dipole excitations, has enjoyed much success in understanding the thermodynamic, static, and transport properties of the so-called spin-ice materials. The artificial version of spin ice, where a lattice of nanoscale magnetic dipoles is sculpted out of a ferromagnetic film, provides a unique opportunity to study these unusual quasiparticles in a material-by-design approach. Here we show that the elementary excitations in the ice phase of a nano-magnetic array arranged in the pentagonal lattice are composite objects comprised of the emergent monopole and a surrounding cloud of opposite uncompensated magnetic charges.

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arXiv (Cornell University), Jul 3, 2023

Topology - Recent Advances and Applications [Working Title]

This chapter reviews the implications of topology in the static and dynamics of magnetic systems.... more This chapter reviews the implications of topology in the static and dynamics of magnetic systems. Our focus is twofold. In the first part, we describe how the application of topology allows an understanding of the structure and dynamics of magnetic textures that separate different magnetic domains in magnetic materials. Topological textures are rationalized in terms of elementary topological defects that determine complex magnetic orders and magnetization dynamics processes in the underlying magnetic systems. The second part studies topological phases and topological phenomena associated with the band theory of linear magnetic excitations. Topological spin waves are usually accompanied by exotic phenomena in magnetic materials such as the emergence of chiral edge states and the magnon Hall effect.

Nature Communications, Oct 17, 2017

Geometrical frustration occurs when entities in a system, subject to given lattice constraints, a... more Geometrical frustration occurs when entities in a system, subject to given lattice constraints, are hindered to simultaneously minimize their local interactions. In magnetism, systems incorporating geometrical frustration are fascinating, as their behavior is not only hard to predict, but also leads to the emergence of exotic states of matter. Here, we provide a first look into an artificial frustrated system, the dipolar trident lattice, where the balance of competing interactions between nearest-neighbor magnetic moments can be directly controlled, thus allowing versatile tuning of geometrical frustration and manipulation of ground state configurations. Our findings not only provide the basis for future studies on the low-temperature physics of the dipolar trident lattice, but also demonstrate how this frustration-by-design concept can deliver magnetically frustrated metamaterials.

Bulletin of the American Physical Society, Mar 16, 2010

Submitted for the MAR10 Meeting of The American Physical Society Conical surfaces and singulariti... more Submitted for the MAR10 Meeting of The American Physical Society Conical surfaces and singularities in highly constrained elastic membranes 1 PAULA MELLADO, SHENGFENG CHENG, ANDRES CONCHA-An elastic membrane that is forced to reside in a container of a slightly smaller size will deform and, upon further volume reduction, will eventually crumple. Previous studies have focused on the onset of the crumpled state by analyzing the mechanical response and stability of a developable conical surface (d-cones) that can be described by a single-valued function, while others have simulated the highly packed regime, neglecting the importance of connectivity of the membrane. Here we present a study in which experiments, numerical simulation and analytic work are used to show that the emergence of new regions of high stretching is a generic outcome when a self-avoiding membrane is subject to a severe geometrical constraint. Consequently, an anomalous mechanical response, characterized by a series of peaks in the force-deformation curve, appears as the membrane is squeezed. Our findings emphasize the role of self-avoidance, connectivity and friction as the key factors defining the morphology and response of a d-cone from its formation to its final fate.

Bulletin of the American Physical Society, Mar 18, 2009

interactions in spin ice are described most effectively in terms of magnetic charges residing on ... more interactions in spin ice are described most effectively in terms of magnetic charges residing on the dual lattice [1]. While spin ice on the pyrochlore lattice contains no magnetic monopoles at low temperatures, spin ice on kagome [2] contains a unit magnetic charge (±1) on every triangle. With the aid of Monte-Carlo simulations, we show that long-range Coulomb interaction between the monopoles lifts the degeneracy of the spin-ice states and induces a phase transition into a state with ordered magnetic charges but no spin order. The residual entropy is reduced from the spinice value but remains extensive. The phase transition is continuous with critical exponents close to the two-dimensional Ising universality class.

Bulletin of the American Physical Society, 2016

The rapid squirt of a proteinaceous slime jet endow velvet worms (Onychophora) with a unique mech... more The rapid squirt of a proteinaceous slime jet endow velvet worms (Onychophora) with a unique mechanism for defense from predators and for capturing prey by entangling them in a disordered web that immobilizes their target. However, to date neither qualitative nor quantitative descriptions have been provided for this unique adaptation. We have investigated the mechanism that allows velvet worms the fast oscillatory motion of their oral papillae and the exiting liquid jet that oscillates with frequencies f ∼ 30 − 60 Hz. Using anatomical images and high speed videography, we show that even without fast muscular action of the papilla, a strong contraction of the slime reservoir and the geometry of the reservoir-papilla system suffices to accelerate the slime to speeds up to v ∼ 5 m/s in about ∆t ∼ 60 ms. A theoretical analysis and a physical simulacrum allow us to infer that this fast oscillatory motion is the result of an elastohydrodynamic instability driven by the interplay between the elasticity of oral papillae and the fast unsteady flow during squirting. We propose several applications that can be implemented using this instability, ranging from high-throughput droplet production, printing, and micro-nanofiber production among others. 1 A.C was partially supported by Fondecyt grant 11130075.

Bulletin of the American Physical Society, 2017