arXiv subjects

Carles Navau

Publications and source records attributed to Carles Navau.

At least 19 recordsLinked to original sources

arXiv2026

Layer- and Field-Dependent Magnetic Order in 2D CrSBr Revealed by Pulsed Nanocalorimetry

Understanding the evolution of magnetic order in the two-dimensional limit remains a central challenge in van der Waals magnets, where thermodynamic measurements are constrained by the femtogram-scale mass of exfoliated flakes. Here, we use microsecond pulse-heating nanocalorimetry to measure the heat capacity and magnetic entropy of CrSBr flakes down to the monolayer limit. The measurements reveal the entropy landscape associated with magnetic ordering, uncovering a reduction of the interlayer transition temperature and an entropy-derived effective magnetic moment approaching the monolayer limit. Thermodynamic anomalies capture a crossover from bulk-like interlayer antiferromagnetism to a regime dominated by intralayer ferromagnetic correlations. A pronounced layer-parity effect further emerges, with odd-layer samples displaying an additional high-temperature contribution associated with uncompensated magnetic layers. Under in-plane magnetic fields applied along the easy axis, antiferromagnetic order is progressively suppressed, allowing extraction of a thickness-dependent critical field reflecting weakened interlayer exchange coupling. Entropy analysis further reveals an extended regime of magnetic fluctuations persisting well above the interlayer ordering transition. Together, these results establish nanocalorimetry as a powerful thermodynamic probe of low-dimensional magnetism, providing direct access to magnetic entropy, exchange interactions, and dimensional crossover in atomically thin van der Waals magnets.

cond-mat.mtrl-sci↗

arXiv2026

Macroscopic Quantum Interference of the Center-of-Mass Motion of Levitated Superconducting Microparticles enabled by Magnetic Higher-Order Traps

We show how magnetostatic higher-order multipole traps can be used to generate macroscopic quantum interference of the motion of levitated superconducting microparticles. An appropriate combination of multipolar magnetic fields offers great versatility in constructing various trap potentials, including anharmonic trap potentials such as Duffing or double-well types. Crucially, the anharmonic trap potentials realize a nonlinearity on the order of hundred times the zero-point motion, i.e., on a length scale below nanometers. These anharmonic potentials allow for the generation of quantum features of the center-of-mass motion of a magnetically levitated superconducting microparticle. Importantly, they can be easily generated with a static arrangement of coils, requiring only that the current running through them is tunable. We propose protocols exploiting the versatility of the magnetic trap landscape to generate non-Gaussian motional states. We solve the dynamics of the center-of-mass motion of the particle in phase space and analyze its parameter dependence. Furthermore, we give a recipe to distinguish classical from quantum behavior in a statistically meaningful way through measurement of the position of the particle. Our results open a path to accessing the quantum regime of the center-of-mass motion of objects with masses larger than picogram, i.e., $10^{13}$ atomic mass units. This will enable fundamental physics experiments for studying the transition between quantum and classical behavior, exploring the intersection between quantum physics and gravity as well as probing of certain types of dark matter.

quant-ph↗

arXiv2026

Magnetic Omniconversion: Source-Independent Molding of Magnetostatic Fields

Magnetic fields are constrained by the geometry and location of their sources, limiting the ability to freely tailor their spatial distribution. We introduce a general framework to passively convert the magnetic field generated by arbitrary sources into any prescribed desired field within a finite source-free region. Our method relies on field shaping using linear magnetic materials, enabling source-independent magnetic-field molding. We provide the general recipe, analytical and numerical demonstrations for some paradigmatic examples, and a proof-of-concept experiment that validates the idea and materials implementation. This approach enables novel possibilities in magnetic shielding, targeted field delivery, advanced imaging technologies, and a broad range of field-control applications.

physics.app-ph↗

arXiv2025

Magnetic flux threading a planar holed superconductor

We analytically evaluate the currents in a planar-holed ideal (perfect magnetic shielding) superconductor when an axial dipole is located along the system's axis, obtaining a simple relation between the fluxoid (in this geometry, it equals the flux) threading the hole, the net current circling it, and the magnitude and position of the dipole. This expression has to be added to the basic screening equations describing the superconductor to understand the different physical situations that can appear. Thus several scenarios are studied: field and zero-field cooling cases, where the flux threading the hole is fixed; superconductors with a slit where the net current circling the hole is zero; and self-inductance case, where the flux threading the hole is only due to the screening currents. All the presented results are analytical for the axial dipole and are given as a function of the distance of the dipole from the plane of the superconductor. The obtained results can serve as a benchmark for recent and ongoing micromechanical experiments aimed at developing ultrasensitive gravimeters or applications searching for exotic spin-dependent interactions and dark matter, for example.

physics.app-ph↗

arXiv2024

Modelling magnetically-levitated superconducting ellipsoids, cylinders and cuboids for quantum magnetomechanics

We theoretically investigate the properties of magnetically-levitated superconducting rotors confined in anti-Helmholtz traps, for application in magnetomechanical experiments. We study both the translational modes and a librational mode. The librational mode gives an additional degree of freedom that levitated spheres do not have access to. We compare rotors of different shapes: ellipsoids, cylinders and cuboids. We find that the stable orientations of the rotors depend on the rotors' aspect ratios.

physics.app-ph↗

arXiv2020

Levitation of superconducting micro-rings for quantum magnetomechanics

Levitation of superconductors is becoming an important building block in quantum technologies, particularly in the rising field of magnetomechanics. In most of the theoretical proposals and experiments, solid geometries such as spheres are considered for the levitator. Here we demonstrate that replacing them by superconducting rings brings two important advantages: Firstly, the forces acting on the ring remain comparable to those expected for solid objects, while the mass of the superconductor is greatly reduced. In turn, this reduction increases the achievable trap frequency. Secondly, the flux trapped in the ring by in-field cooling yields an additional degree of control for the system. We construct a general theoretical framework with which we obtain analytical formulations for a superconducting ring levitating in an anti-Helmholtz quadrupole field and a dipole field, for both zero-field and in-field cooling. The positions and the trapping frequencies of the levitated rings are analytically found as a function of the parameters of the system and the field applied during the cooling process. Unlike what is commonly observed in bulk superconductors, lateral and rotational stability are not granted for this idealized geometry. We therefore discuss the requirements for simple superconducting structures to achieve stability in all degrees of freedom.

cond-mat.supr-con↗

arXiv2019

Accelerating, guiding, and compressing skyrmions by defect rails

Magnetic skyrmions are promising candidates as information carriers in spintronic devices. The transport of individual skyrmions in a fast and controlled way is a key issue in this field. Here we introduce a novel platform for accelerating, guiding and compressing skyrmions along predefined paths. The guiding mechanism is based on two parallel defect-lines (rails), one attractive and the other repulsive. Numerical simulations, using parameters from state-of-the-art experiments, show that the speed of the skyrmions along the rails is increased up to a factor of ten with respect to the non-defect case whereas the distance between rails can be as small as the initial radius of the skyrmions. In this way, the flux of information that can be coded and transported with magnetic skyrmions could be significantly increased.

cond-mat.mes-hall↗

arXiv2018

Magnetic illusion: transforming a magnetic object into another object by negative permeability

We theoretically predict and experimentally verify the illusion of transforming the magnetic signature of a 3D object into that of another arbitrary object. This is done by employing negative-permeability materials, which we demonstrate that can be emulated by tailored sets of currents. The experimental transformation of the magnetic response of a ferromagnet into that of its antagonistic material, a superconductor, is presented to confirm the theory. The emulation of negative-permeability materials by currents provides a new pathway for designing devices for controlling magnetic fields in unprecedented ways.

cond-mat.mtrl-sci↗

arXiv2018

Negative permeability in magnetostatics and its experimental demonstration

The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negative-permeability values. The understanding of these new solutions allow us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negative-permeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical applications.

cond-mat.mtrl-sci↗

arXiv2015

New Reversal Mode in Exchange Coupled Antiferromagnetic/Ferromagnetic Disks: Distorted Viscous Vortex

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. Additionally, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir20Mn80/Fe20Ni80 microdots: distorted viscous vortex reversal. Contrary to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.

cond-mat.mtrl-sci↗

arXiv2014

Long-Distance Transfer and Routing of Static Magnetic Fields

We show how the static magnetic field of a finite source can be transferred and routed to arbitrary long distances. This is achieved by using transformation optics, which results in a device made of a material with a highly anisotropic magnetic permeability. We show that a simplified version of the device, made by a superconducting-ferromagnet hybrid, also leads to an excellent transfer of the magnetic field. The latter is demonstrated with a proof-of-principle experiment where a ferromagnet tube coated with a superconductor improves the transfer of static magnetic fields with respect to conventional methods by a 400\% factor over distances of 14cm.

cond-mat.supr-con↗

arXiv2014

Imprinting skyrmions in thin films by ferromagnetic and superconducting templates

Magnetic skyrmions are promising candidates as information carriers in a new generation of memories. How to generate and stabilize skyrmions is essential for their successful application to technology. Here we theoretically demonstrate that arrays of skyrmions can be imprinted in ultrathin ferromagnetic films in large numbers by bringing a magnetic nanostructured template close to the film. Two kind of templates, allowed by present-day nanotechnologies, are studied: arrays of ferromagnetic nanorods and superconducting vortices. Skyrmions are generated when exposing magnetic films to the template fields for short times and remain stable after removing the template.

cond-mat.mtrl-sci↗

arXiv2013

Experimental realization of magnetic energy concentration and transmission at a distance by metamaterials

Controlling electromagnetic energy is essential for an efficient and sustainable society. A key requirement is concentrating magnetic energy in a desired volume of space in order to either extract the energy to produce work or store it. Metamaterials have opened new possibilities for controlling electromagnetic energy. Recently, a superconductor-ferromagnetic metamaterial that allows unprecedented concentration and amplification of magnetic energy, and also its transmission at distance through free space, has been devised theoretically. Here we design and build an actual version of the superconductor-ferromagnetic metamaterial and experimentally confirm these properties. We show that also a ferromagnetic metamaterial, without superconducting parts, can achieve concentration and transmission of energy with only a slight decrease in the performance. Transmission of magnetic energy at a distance by magnetic metamaterials may provide new ways of enhancing wireless power transmission, where efficiency depends critically on the magnetic coupling strength between source and receiver.

cond-mat.supr-con↗

arXiv2012

Magnetic energy harvesting and concentration at distance by transformation optics

Magnetic energy is one the main agents powering our society: generating energy in power plants, keeping information in magnetic memories, moving our devices with motors. All of these applications require a certain spatial distribution of magnetic energy, for example concentrating it in a transformer core or in a magnetic sensor. We introduce in this work a way to collect magnetic energy and distribute it in space with unprecedented efficiency and flexibility, allowing very large concentration of magnetic energy in a free space region, an enhanced magnetic coupling between two magnetic sources, and the transfer of magnetic energy from a source to a given distant point separated by empty space. All these features are achieved with a single device, a magnetic shell designed by transformation optics.

physics.optics↗

arXiv2011

Exact cloaking of uniform magnetic fields with a superconductor-magnetic bilayer

The experimental realization of cloaks for electromagnetic fields has been possible only in reduced schemes because of extreme material conditions required in exact cloaks. Here we analytically demonstrate that a simple arrangement of two concentric homogeneous and isotropic layers with different permeabilities can keep a uniform externally applied magnetic field unperturbed. If the inner layer is superconducting, the device prevents exterior fields from entering an enclosed region, which represents a case for an exact cloaking of uniform fields. When the applied field is inhomogeneous the cloaking property is reduced but may be optimized by changing the thickness of the layers.

cond-mat.supr-con↗

arXiv2011

Two-dimensional arrays of superconducting strips as dc magnetic metamaterials

We theoretically investigate the magnetic response of two-dimensional arrays of superconducting strips, which are regarded as essential structures of dc magnetic metamaterials. We analytically obtain local distributions of the magnetic field for the ideal complete shielding state (i.e., $Λ/w\to 0$, where $2w$ is the strip width, $Λ=λ^2/d$ is the Pearl length, $λ$ is the London penetration depth, and $d$ is the strip thickness), and derive effective permeability by averaging the local field distributions. We also perform numerical calculations for a realistic case, taking finite $Λ/w>0$ into account. We investigate two types of strip arrays: a rectangular array and a hexagonal array. The resulting effective permeability has large anisotropy that depends on the dimensions and arrangement of the superconducting strips, and the hexagonal array is found to be more advantageous for obtaining large anisotropy than the rectangular array.

cond-mat.supr-con↗

arXiv2011

Antimagnets: Controlling magnetic fields with superconductor-metamaterial hybrids

Magnetism is very important in science and technology, from magnetic recording to energy generation to trapping cold atoms. Physicists have managed to master magnetism - to create and manipulate magnetic fields- almost at will. Surprisingly, there is at least one property which until now has been elusive: how to 'switch off' the magnetic interaction of a magnetic material with existing magnetic fields without modifying them. Here we introduce the antimagnet, a design to conceal the magnetic response of a given volume from its exterior, without altering the external magnetic fields, somehow analogous to the recent theoretical proposals for cloaking electromagnetic waves with metamaterials. However, different from these devices requiring extreme material properties, our device is feasible and needs only two kinds of available materials: superconductors and isotropic magnetic materials. Antimagnets may have applications in magnetic-based medical techniques such as MRI or in reducing the magnetic signature of vessels or planes.

cond-mat.supr-con↗

arXiv2009

Self-fields in thin superconducting tapes: implications to the thickness effect in coated conductors

Most applications of superconductors, such as power transmission lines, motors, generators, and transformers, require long cables through which large currents circulate. Impressive progress has recently been achieved in the current-carrying capability in conductors based on high-temperature superconductors. Coated conductors are likely the best examples, consisting of very good quality thin layers of YBCO superconductor grown on top of a metallic tape with some intermediate layers. However, there is an important problem for achieving large currents: a large decrease in transport critical-current density Jc when increasing film thickness has been observed in coated conductors made by all available techniques. Here, we theoretically explain the nature and the ubiquitous presence of this so-called thickness effect by analyzing the self-field created by the transport currents in the superconductor, assuming a realistic field-dependent Jc. This knowledge can help finding new ways to improve transport current in thick superconducting films.

cond-mat.supr-con↗