arXiv subjects

Mauricio Sturla

Publications and source records attributed to Mauricio Sturla.

9 recordsLinked to original sources

arXiv2024

Conformal Boundary as Holographic Dual to the Hyperbolic Fracton Model

In addition to describing our universe, gravitational theories profoundly inspire the study of emergent properties of exotic phases of matter. While the Anti-de Sitter/conformal field theory (AdS/CFT) correspondence is one of the most celebrated examples, the field of fractonic matter -- driven in part by gapless phases resembling linearized gravity -- has also seen rapid developments. Despite the deep implications of both areas, connections between them remain sparse, primarily due to the difficulty in constructing explicit models that encapsulate both fields' essential features. Here we demonstrate the efficacy of the recently proposed Hyperbolic Fracton Model as a concrete model for AdS/CFT duality. Using explicit numerical and analytical calculations on the discrete hyperbolic lattice, we show that the boundary state exhibits conformal field theory properties. Our main result is that bulk defects induce an emergent temperature for the boundary state, proportional to the defect perimeter, in quantitative agreement with the expected behaviour of a black hole in AdS spacetime. The Hyperbolic Fracton Model thus emerges as a unique lattice model of holographic principle equipped with a well-defined bulk Hamiltonian, and offers a promising gateway for studying a wide range of holographic phenomena.

cond-mat.str-el↗

arXiv2024

Quantum Fluctuations in the van der Walls material $\rm NiPS_3$

We present the magnetic excitation spectrum of the quantum magnet $\rm NiPS_3$ near the zig-zag ground state of a minimal honeycomb spin Hamiltonian that includes bilinear and biquadratic spin interactions. Our analysis, using a multi-boson generalized spin wave theory suited for spin S=1 systems, revealed two normal modes at the linear level. The one at lower energy corresponds to a single magnon mode, consistent with results from spectroscopy experiments. Without single-ion anisotropy, this mode features a Goldstone mode at the corner of the Brillouin zone. When single ion anisotropy is introduced, the zig-zag phase's global U(1) invariance is broken, resulting in a gap. The higher energy mode corresponds to two-magnon fluctuations, which appear at the harmonic level in the generalized spin wave theory. This mode forms a gapped flat band due to bilinear spin interactions and becomes dispersive when biquadratic interactions are considered. The higher energy dispersion is related to quadrupolar fluctuations, which are feasible in magnets where the order parameter fluctuates in the SU(3) space. The spectrum analysis yielded quantum corrections to the order parameter and detected instabilities in the $\rm NiPS_3$ dipolar phases. Identifying the highest energy branch in experiments could provide insight into hidden nematic orders in $\rm NiPS_3$ and other van der Waals magnets.

cond-mat.str-el↗

arXiv2024

Perfectly hidden order and Z2 confinement transition in a fully packed monopole liquid

We investigate a variant of spin ice whose degenerate ground states are densely packed monopole configurations. An applied field drives this model through a Z2 confinement transition. This phase change is a variant of the U(1) Kasteleyn transition, but instead of saturated order the system has fluctuations in the confined phase, and shows critical scaling on both sides of the transition. Remarkably, the magnetic response scales with the critical exponent of the specific heat in the 3D Ising universality class. We prove this universality using a Kramers--Wannier duality to map to an Ising model. The dual order parameter maps back to a non-local string order parameter in the original model, invisible to any local probe. Further, we describe the transition in terms of a bosonic field theory including a pairing term.

cond-mat.str-el↗

arXiv2016

Approximate analytic expression for the Skyrmions crystal

We find approximate solutions for the two-dimensional non-linear Σ-model with Dzyalioshinkii-Moriya term, representing magnetic Skyrmions. They are built in an analytic form, by pasting different approximate solutions found in different regions of space. We verify that our construction reproduces the phenomenology known from numerical solutions and Monte Carlo simulations, giving rise to a Skyrmion lattice at an intermediate range of magnetic field, flanked by spiral and ferromagnetic phases for low and high magnetic field respectively.

cond-mat.str-el↗

arXiv2015

Generalized effective hamiltonian for graphene under non-uniform strain

We use a symmetry approach to construct a systematic derivative expansion of the low energy effective Hamiltonian modifying the continuum Dirac description of graphene in the presence of non-uniform elastic deformations. We extract all experimentally relevant terms and describe their physical significance. Among them there is a new gap-opening term that describes the Zeeman coupling of the elastic pseudomagnetic field and the pseudospin. We determine the value of the couplings using a generalized tight binding model.

cond-mat.mes-hall↗

arXiv2014

Electronic structure of spontaneously strained graphene on hexagonal Boron Nitride

Hexagonal Boron Nitride substrates have been shown to dramatically improve the electric properties of graphene. Recently, it has been observed that when the two honeycomb crystals are close to perfect alignment, strong lattice distortions develop in graphene due to the moiré adhesion landscape. Simultaneously a gap opens at the Dirac point. Here we derive a simple low energy model for graphene carriers close to alignment with the substrate, taking into account spontaneous strains at equilibrium, pseudogauge fields and deformation potentials. We carry out a detailed characterisation of the modified band structure, gap, local and global density of states, and band topology in terms of physical parameters. We show that the overall electronic structure is strongly modified by the spontaneous strains.

cond-mat.mes-hall↗

arXiv2014

Spontaneous Strains and Gap in Graphene on Boron Nitride

The interaction between a graphene layer and a hexagonal Boron Nitride (hBN) substrate induces lateral displacements and strains in the graphene layer. The displacements lead to the appearance of commensurate regions and the existence of an average gap in the electronic spectrum of graphene. We present a simple, but realistic model, by which the displacements, strains and spectral gap can be derived analytically from the adhesion forces between hBN and graphene. When the lattice axes of graphene and the substrate are aligned, strains reach a value of order 2\%, leading to effective magnetic fields above 100T. The combination of strains and induced scalar potential gives a sizeable contribution to the electronic gap. Commensuration effects are negligible, due to the large stiffness of graphene.

cond-mat.mes-hall↗

arXiv2012

Space dependent Fermi velocity in strained graphene

We resolve some apparent discrepancies between two different models for curved graphene: the one based on tight binding and elasticity theory, and the covariant approach based on quantum field theory in curved space. We demonstrate that strained or corrugated samples will have a space dependent Fermi velocity in either approach that can affect the interpretation of some experiments.

cond-mat.mes-hall↗

arXiv2010

Wormhole solutions to Horava gravity

We present wormhole solutions to Horava non-relativistic gravity theory in vacuum. We show that, if the parameter $λ$ is set to one, transversable wormholes connecting two asymptotically de Sitter or anti-de Sitter regions exist. In the case of arbitrary $λ$, the asymptotic regions have a more complicated metric with constant curvature. We also show that, when the detailed balance condition is violated softly, tranversable and asymptotically Minkowski, de Sitter or anti-de Sitter wormholes exist.

hep-th↗