arXiv subjects

Paula Mellado

Publications and source records attributed to Paula Mellado.

32 records · Page 2Linked to original sources

arXiv2019

Photoinduced Floquet topological magnons in Kitaev magnets

We study periodically driven pure Kitaev model and ferromagnetic phase of the Kitaev-Heisenberg model on the honeycomb lattice by off-resonant linearly and circularly-polarized lights at zero magnetic field. Using a combination of linear spin wave and Floquet theories, we show that the effective time-independent Hamiltonians in the off-resonant regime map onto the corresponding anisotropic static spin model, plus a tunable photoinduced magnetic field along the $[111]$ direction, which precipitates Floquet topological magnons and chiral magnon edge modes. They are tunable by the light amplitude and polarization. Similarly, we show that the thermal Hall effect induced by the Berry curvature of the Floquet topological magnons can also be tuned by the laser field. Our results pave the way for ultrafast manipulation of topological magnons in irradiated Kitaev magnets, and could play a pivotal role in the investigation of ultrafast magnon spin current generation in Kitaev materials.

cond-mat.str-el↗

arXiv2018

Charge-transfer insulation in twisted bilayer graphene

We studied the real space structure of states in twisted bilayer graphene at the `magic angle' $θ= 1.08^\circ$. 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 $p+ip$-wave whereas electron-doping yields $d+id$-wave pairing symmetry.

cond-mat.str-el↗

arXiv2018

Designing hysteresis with dipolar chains

Materials that have a hysteretic response to an external field are essential in modern information storage and processing technologies. The magnetization curves of several natural and artificial materials have previously been measured and explained in terms of the Neel model, Preisach phenomenological model, the creation, propagation, and annihilation of topological defects, or by resonant quantum tunnelling of the magnetization in single molecule magnets. However, a simple way to design the magnetic response of a material is missing. Here, we propose and experimentally realize an elementary method to engineer hysteresis loops in metamaterials built out of dipolar chains. We show that by tuning the system's interactions and geometry we can induce magnetic responses with or without remanence at will. Our findings pave the way for the rational design of hysteretical responses in a variety of physical systems such as dipolar cold atoms, ferroelectrics, or artificial magnetic lattices, among others.

physics.app-ph↗

arXiv2015

Projective symmetry of partons in Kitaev's honeycomb model

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.

cond-mat.str-el↗

arXiv2014

How the velvet worm squirts slime

The rapid squirt of a proteinaceous slime jet endows the ancient 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. Here we investigate the fast oscillatory motion of the oral papillae and the exiting liquid jet that oscillates with frequencies $f\sim 30-60$ Hz. Using anatomical images, high speed videography, theoretical analysis and a physical simulacrum we show 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. Our results demonstrate how passive strategies can be cleverly harnessed by organisms, while suggesting future oscillating micro-fluidic devices as well as novel ways for micro and nano fiber production using bioinspired strategies.

physics.flu-dyn↗

arXiv2014

Unpaired Majorana modes on dislocations and string defects in Kitaev's honeycomb model

We study the gapped phase of Kitaev's honeycomb model (a $Z_2$ spin liquid) on a lattice with topological defects. We find that some dislocations and string defects carry unpaired Majorana fermions. Physical excitations associated with these defects are (complex) fermion modes made out of two (real) Majorana fermions connected by a $Z_2$ gauge string. The quantum state of these modes is robust against local noise and can be changed by winding a $Z_2$ vortex around one of the dislocations. The exact solution respects gauge invariance and reveals a crucial role of the gauge field in the physics of Majorana modes. To facilitate these theoretical developments, we recast the degenerate perturbation theory for spins in the language of Majorana fermions.

cond-mat.str-el↗

arXiv2013

Unpaired Majorana modes in the gapped phase of Kitaev's honeycomb model

We study the gapped phase of Kitaev's honeycomb model (a Z_2 spin liquid) in the presence of lattice defects. We find that some dislocations and bond defects carry unpaired Majorana fermions. Physical excitations associated with these defects are (complex) fermion modes made out of two (real) Majorana fermions connected by a Z_2 gauge string. The quantum state of these modes is robust against local noise and can be changed by winding a Z_2 vortex around a dislocation. The exact solution respects gauge invariance and reveals a crucial role of the gauge field in the physics of Majorana modes.

cond-mat.str-el↗

arXiv2013

Magnetic monopole polarons in spin ice with mixed coordination numbers

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.

cond-mat.str-el↗

arXiv2012

Macroscopic magnetic frustration

Although geometrical frustration transcends scale, it has primarily been evoked in the micro and mesoscopic realm to characterize such phases as spin-ice liquids and glasses and to explain the behavior of such materials as multiferroics, high temperature superconductors, colloids and copolymers. Here we introduce a system of macroscopic ferromagnetic rotors arranged in a planar lattice capable of out-of-plane movement that exhibit the characteristic honeycomb spin ice rules studied and seen so far only in its mesoscopic manifestation. We find that a polarized initial state of this system settles into the honeycomb spin ice phase with relaxation on multiple time scales. We explain this relaxation process using a minimal classical mechanical model which includes Coulombic interactions between magnetic charges located at the ends of the magnets and viscous dissipation at the hinges. Our study shows how macroscopic frustration arises in a purely classical setting that is amenable to experiment, easy manipulation, theory and computation, and shows phenomena that are not visible in their microscopic counterparts.

cond-mat.mtrl-sci↗

arXiv2012

Dynamics of artificial spin ice: continuous honeycomb network

We model the dynamics of magnetization in an artificial analog of spin ice specializing to the case of a honeycomb network of connected magnetic nanowires. The inherently dissipative dynamics is mediated by the emission, propagation and absorption of domain walls in the links of the lattice. These domain walls carry two natural units of magnetic charge, whereas sites of the lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between these charges play a major role in the physics of the system, as does quenched disorder caused by imperfections of the lattice. We identify and describe different regimes of magnetization reversal in an applied magnetic field determined by the orientation of the applied field with respect to the initial magnetization. One of the regimes is characterized by magnetic avalanches with a 1/n distribution of lengths.

cond-mat.mes-hall↗

arXiv2012

A simple model for nanofiber formation by rotary jet-spinning

Nanofibers are microstructured materials that span a broad range of applications from tissue engineering scaffolds to polymer transistors. An efficient method of nanofiber production is Rotary Jet-Spinning (RJS), consisting of a perforated reservoir rotating at high speeds along its axis of symmetry, which propels a liquid, polymeric jet out of the reservoir orifice. These jets undergo stretching and solidify forming nanoscale fibers. We report a minimal scaling framework complemented by a semi-analytic and numerical approach to characterize the regimes of nanofiber production using RJS. Our theoretical model is validated for the fiber radius as a function of experimentally tunable parameters. We summarize our findings in a phase diagram for the design space of continuous nanofibers as a function of process parameters, in good agreement with experiments and with natural implications for the production rates as well as in the morphological quality of fibers.

cond-mat.mtrl-sci↗

arXiv2011

Two-stage ordering of spins in a dipolar spin ice on the kagome lattice

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.

cond-mat.str-el↗

arXiv2011

Mechanical response of a self avoiding membrane: fold collisions and the birth of conical singularities

An elastic membrane that is forced to reside in a container smaller than its natural size will deform and, upon further volume reduction, eventually crumple. The crumpled state is characterized by the localization of energy in a complex network of highly deformed crescent-like regions joined by line ridges. Previous studies have focused on the onset of the crumpled state by analyzing the mechanical response and stability of a conical dislocation, while others have simulated the highly packed regime neglecting the importance of the connectivity of the membrane. Here we show, through a combination of experiments, numerical simulations, and analytic approach, 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. We demonstrate that, at moderate packing fraction, interlayer interactions produce a response equivalent to the one of a thicker membrane that has the shape of the deformed one. Evidence is found that friction plays a key role stabilizing the folded structures.

cond-mat.mtrl-sci↗

arXiv2010

Dynamics of magnetic charges in artificial spin ice

Artificial spin ice has been recently implemented in two-dimensional arrays of mesoscopic magnetic wires. We propose a theoretical model of magnetization dynamics in artificial spin ice under the action of an applied magnetic field. Magnetization reversal is mediated by domain walls carrying two units of magnetic charge. They are emitted by lattice junctions when the the local field exceeds a critical value $H_c$ required to pull apart magnetic charges of opposite sign. Positive feedback from Coulomb interactions between magnetic charges induces avalanches in magnetization reversal.

cond-mat.mtrl-sci↗