arXiv subjects

M. J. Martínez-Pérez

Publications and source records attributed to M. J. Martínez-Pérez.

15 recordsLinked to original sources

arXiv2025

Strong spin-magnon coupling in a van der Waals magnet with tunable chiral symmetry

Quantum technologies require platforms that can coherently interface qubits with bosonic excitations. Photons have traditionally played this role in cavity quantum electrodynamics, but achieving the same goal using solid-state bosons remains challenging. Here we demonstrate strong and tunable spin-magnon coupling between molecular spin qubits and magnons in a van der Waals antiferromagnetic insulator. Using [Gd(W$_5$O$_{18}$)$_{2}$]$^{9-}$ as the spin ensemble and CrSBr as the magnonic resonator, we observe anticrossings and coherent hybridization, realizing magnon quantum electrodynamics for the first time. Crucially, by rotating the magnetic field, we can dynamically change the magnon symmetry from linear to chiral, enabling in-situ tuning of the coupling strength. Our results establish magnonic cavities as a platform for scalable chiral quantum optics with magnetic materials.

quant-ph↗

arXiv2024

Metastability in individual magnetic vortices

Magnetic nanoparticles play a crucial role in different fields such as biomedicine or information and quantum technologies. These applications require nanoparticles with a single, well-defined energy minimum, free of metastable states, and characterized by narrow switching field distributions. Here, we demonstrate that high-transition-temperature nanoSQUIDs can be successfully applied to the characterization of individual nanodiscs hosting magnetic vortices. We present measurements performed under varying temperature and external magnetic field, revealing signatures of ubiquitous, multiple metastable configurations. We also demonstrate that metastability can be reduced by introducing an intended asymmetry. NanoSQUID measurements can be applied to optimize the fabrication of on-demand spin-texture states, such as degenerated vortices or particles with fixed circulation and deterministic and narrow switching probabilities.

cond-mat.mes-hall↗

arXiv2021

Quantum Size Effects in the Magnetic Susceptibility of a Metallic Nanoparticle

We theoretically study quantum size effects in the magnetic response of a spherical metallic nanoparticle (e.g. gold). Using the Jellium model in spherical coordinates, we compute the induced magnetic moment and the magnetic susceptibility for a nanoparticle in the presence of a static external magnetic field. Below a critical magnetic field the magnetic response is diamagnetic, whereas above such field the magnetization is characterized by sharp, step-like increases of several tenths of Bohr magnetons, associated with the Zeeman crossing of energy levels above and below the Fermi sea. We quantify the robustness of these regimes against thermal excitations and finite linewidth of the electronic levels. Finally, we propose two methods for experimental detection of the quantum size effects based on the coupling to superconducting quantum interference devices.

cond-mat.mes-hall↗

arXiv2020

Enhanced molecular spin-photon coupling at superconducting nanoconstrictions

We combine top-down and bottom-up nanolithography to optimize the coupling of small molecular spin ensembles to $1.4$ GHz on-chip superconducting resonators. Nanoscopic constrictions, fabricated with a focused ion beam at the central transmission line, locally concentrate the microwave magnetic field. Drops of free-radical molecules have been deposited from solution onto the circuits. For the smallest ones, the molecules were delivered at the relevant circuit areas by means of an atomic force microscope. The number of spins $N_{\rm eff}$ effectively coupled to each device was accurately determined combining Scanning Electron and Atomic Force Microscopies. The collective spin-photon coupling constant has been determined for samples with $N_{\rm eff}$ ranging between $2 \times 10^{6}$ and $10^{12}$ spins, and for temperatures down to $44$ mK. The results show the well-known collective enhancement of the coupling proportional to the square root of $N_{\rm eff}$. The average coupling of individual spins is enhanced by more than four orders of magnitude (from $4$ mHz up to above $180$ Hz) when the transmission line width is reduced from $400$ microns down to $42$ nm, and reaches maximum values near $1$ kHz for molecules located on the smallest nanoconstrictions. This result opens promising avenues for the realization of magnetic spectroscopy experiments at the nanoscale and for the development of hybrid quantum computation architectures based on molecular spin qubits.

quant-ph↗

arXiv2018

NanoSQUIDs: Basics & recent advances

Superconducting Quantum Interference Devices (SQUIDs) are one of the most popular devices in superconducting electronics. They combine the Josephson effect with the quantization of magnetic flux in superconductors. This gives rise to one of the most beautiful manifestations of macroscopic quantum coherence in the solid state. In addition, SQUIDs are extremely sensitive sensors allowing to transduce magnetic flux into measurable electric signals. As a consequence, any physical observable that can be converted into magnetic flux, e.g., current, magnetization, magnetic field or position, becomes easily accessible to SQUID sensors. In the late 1980's it became clear that downsizing the dimensions of SQUIDs to the nanometric scale would encompass an enormous increase of their sensitivity to localized tiny magnetic signals. Indeed, nanoSQUIDs opened the way to the investigation of, e.g., individual magnetic nanoparticles or surface magnetic states with unprecedented sensitivities. The purpose of this review is to present a detailed survey of microscopic and nanoscopic SQUID sensors. We will start by discussing the principle of operation of SQUIDs, placing the emphasis on their application as ultrasensitive detectors for small localized magnetic signals. We will continue by reviewing a number of existing devices based on different kinds of Josephson junctions and materials, focusing on their advantages and drawbacks. The last sections are left for applications of nanoSQUIDs in the fields of scanning SQUID microscopy and magnetic particle characterization, putting special stress on the investigation of individual magnetic nanoparticles.

cond-mat.supr-con↗

arXiv2016

NanoSQUID magnetometry of individual cobalt nanoparticles grown by focused electron beam induced deposition

We demonstrate the operation of low-noise nano superconducting quantum interference devices (SQUIDs) based on the high critical field and high critical temperature superconductor YBa$_2$Cu$_3$O$_7$ (YBCO) as ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have been patterned by focused ion beam milling. This allows to precisely define the lateral dimensions of the SQUIDs so as to achieve large magnetic coupling between the nanoloop and individual MNPs. By means of focused electron beam induced deposition, cobalt MNPs with typical size of several tens of nm have been grown directly on the surface of the sensors with nanometric spatial resolution. Remarkably, the nanoSQUIDs are operative over extremely broad ranges of applied magnetic field (-1 T $< μ_0 H <$ 1 T) and temperature (0.3 K $< T<$ 80 K). All these features together have allowed us to perform magnetization measurements under different ambient conditions and to detect the magnetization reversal of individual Co MNPs with magnetic moments (1 - 30) $\times 10^6\,μ_{\rm B}$. Depending on the dimensions and shape of the particles we have distinguished between two different magnetic states yielding different reversal mechanisms. The magnetization reversal is thermally activated over an energy barrier, which has been quantified for the (quasi) single-domain particles. Our measurements serve to show not only the high sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these sensors are exceptional magnetometers for the investigation of the properties of individual nanomagnets.

cond-mat.supr-con↗

arXiv2016

Three-Axis Vector Nano Superconducting Quantum Interference Device

We present the design, realization and performance of a three-axis vector nano Superconducting QUantum Interference Device (nanoSQUID). It consists of three mutually orthogonal SQUID nanoloops that allow distinguishing the three components of the vector magnetic moment of individual nanoparticles placed at a specific position. The device is based on Nb/HfTi/Nb Josephson junctions and exhibits linewidths of $\sim 250$ nm and inner loop areas of $600 \times 90$ nm$^2$ and $500 \times 500$ nm$^2$. Operation at temperature $T=4.2$ K, under external magnetic fields up to $\sim 50$ mT is demonstrated. The experimental flux noise below $\sim 250$ ${\rm n}Φ_0/\sqrt{\rm Hz}$ in the white noise limit and the reduced dimensions lead to a total calculated spin sensitivity of $\sim 630$ $μ_{\rm B}/\sqrt{\rm Hz}$ and $\sim 70$ $μ_{\rm B}/\sqrt{\rm Hz}$ for the in-plane and out-of-plane components of the vector magnetic moment, respectively. The potential of the device for studying tridimensional properties of individual nanomagnets is discussed.

cond-mat.supr-con↗

arXiv2015

Rectification of electronic heat current by a hybrid thermal diode

We report the realization of an ultra-efficient low-temperature hybrid heat current rectifier, thermal counterpart of the well-known electric diode. Our design is based on a tunnel junction between two different elements: a normal metal and a superconducting island. Electronic heat current asymmetry in the structure arises from large mismatch between the thermal properties of these two. We demonstrate experimentally temperature differences exceeding $60$ mK between the forward and reverse thermal bias configurations. Our device offers a remarkably large heat rectification ratio up to $\sim 140$ and allows its prompt implementation in true solid-state thermal nanocircuits and general-purpose electronic applications requiring energy harvesting or thermal management and isolation at the nanoscale.

cond-mat.mes-hall↗

arXiv2014

Coherent caloritronics in Josephson-based nanocircuits

We describe here the first experimental realization of a heat interferometer, thermal counterpart of the well-known superconducting quantum interference device (SQUID). These findings demonstrate, on the first place, the existence of phase-dependent heat transport in Josephson-based superconducting circuits and, on the second place, open the way to novel ways of mastering heat at the nanoscale. Combining the use of external magnetic fields for phase biasing and different Josephson junction architectures we show here that a number of heat interference patterns can be obtained. The experimental realization of these architectures, besides being relevant from a fundamental physics point of view, might find important technological application as building blocks of phase-coherent quantum thermal circuits. In particular, the performance of two different heat rectifying devices is analyzed.

cond-mat.supr-con↗

arXiv2013

Non-galvanic primary thermometry of a two-dimensional electron gas

We report the experimental realization of a non-galvanic, primary thermometer capable of measuring the electron temperature of a two-dimensional electron gas with negligible thermal load. Such a thermometer consists of a quantum dot whose temperature-dependent, single-electron transitions are detected by means of a quantum-point-contact electrometer. Its operating principle is demonstrated for a wide range of electron temperatures from 40 to 800 mK. This noninvasive thermometry can find application in experiments addressing the thermal properties of micrometer-scale mesoscopic electron systems, where heating or cooling electrons requires relatively low thermal budgets.

cond-mat.mes-hall↗

arXiv2013

Realization of quantum diffraction of a thermal flux

The first evidence of the dc Josephson effect dates back to 1963 when J. S. Rowell measured the diffraction pattern of the critical current flowing through a single superconducting tunnel junction subjected to an in-plane magnetic field. Interference of Josephson currents through two tunnel junctions connected in parallel was achieved one year later leading to the first ever superconducting quantum interferometer. The latter, together with Rowell's observations, constituted the unequivocal demonstration of the Josephson supercurrent-phase relation. Yet, the Josephson effect has further profound implications going beyond electrical transport, as the interplay between the Cooper condensate and unpaired electrons provides thermal flow through the junction with phase coherence as well. Here we report the first demonstration of quantum diffraction of a heat flux showing that a temperature-biased single Josephson junction is exploited as a diffractor for thermal currents. Specifically, thermal diffraction manifests itself with a peculiar modulation of the electron temperature in a small metallic electrode nearby-contacted to the junction when sweeping the magnetic flux $Φ$. Remarkably, the observed temperature dependence exhibits $Φ$-symmetry and a clear reminiscence with a Fraunhofer-like modulation pattern, as expected fingerprints for a quantum diffraction phenomenon. Our results confirm a pristine prediction of quantum heat transport and, joined with double-junction heat interferometry demonstrated in Nature 492, 401 (2012), exemplify the complementary and conclusive proof of the existence of phase-dependent thermal currents in Josephson-coupled superconductors. This approach combined with well-known methods for phase-biasing superconducting circuits provides with a novel tool for mastering heat fluxes at the nanoscale.

cond-mat.mes-hall↗

arXiv2013

Coupling single molecule magnets to quantum circuits

In this work we study theoretically the coupling of single molecule magnets (SMMs) to a variety of quantum circuits, including microwave resonators with and without constrictions and flux qubits. The main results of this study is that it is possible to achieve strong and ultrastrong coupling regimes between SMM crystals and the superconducting circuit, with strong hints that such a coupling could also be reached for individual molecules close to constrictions. Building on the resulting coupling strengths and the typical coherence times of these molecules (of the order of microseconds), we conclude that SMMs can be used for coherent storage and manipulation of quantum information, either in the context of quantum computing or in quantum simulations. Throughout the work we also discuss in detail the family of molecules that are most suitable for such operations, based not only on the coupling strength, but also on the typical energy gaps and the simplicity with which they can be tuned and oriented. Finally, we also discuss practical advantages of SMMs, such as the possibility to fabricate the SMMs ensembles on the chip through the deposition of small droplets.

cond-mat.mes-hall↗

arXiv2012

Nongalvanic thermometry for ultracold two-dimensional electron domains

Measuring the temperature of a two-dimensional electron gas at temperatures of a few mK is a challenging issue, which standard thermometry schemes may fail to tackle. We propose and analyze a nongalvanic thermometer, based on a quantum point contact and quantum dot, which delivers virtually no power to the electron system to be measured.

cond-mat.mes-hall↗

arXiv2011

Ac magnetic susceptibility of a molecular magnet submonolayer directly patterned onto a microSQUID sensor

We report the controlled integration, via Dip Pen Nanolithography, of monolayer dots of ferritin-based CoO nanoparticles (12 Bohr magnetons) into the most sensitive areas of a microSQUID sensor. The nearly optimum flux coupling between these nanomagnets and the microSQUID improves the achievable sensitivity by a factor 100, enabling us to measure the linear susceptibility of the molecular array down to very low temperatures (13 mK). This method opens the possibility of applying ac susceptibility experiments to characterize two-dimensional arrays of single molecule magnets within a wide range of temperatures and frequencies.

cond-mat.mes-hall↗

arXiv2011

Molecular prototypes for spin-based CNOT quantum gates

We show that a chemically engineered structural asymmetry in [Tb2] molecular clusters renders the two weakly coupled Tb3+ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate.

cond-mat.mes-hall↗