arXiv subjects

Chiara Ciccarelli

Publications and source records attributed to Chiara Ciccarelli.

15 recordsLinked to original sources

arXiv2026

Real-space microscopic description of laser-pulse induced melting of superconductivity

Quenching quantum order via laser pulses has proven a useful tool to access exotic physical effects in systems that are strongly perturbed out of equilibrium. However, theoretical modelling of experimental measurements is typically done phenomenologically or by assuming translational invariance due to the complexity of the problem. Here, we solve a microscopic real-space model of the time dynamics of a superconductor following an intense laser-pulse. We are able to reproduce recent experimental findings displaying a critical slowing-down of the melting of the order parameter for laser fluences close to the condensation energy. Moreover, we leverage the real-space resolution of our model to predict how phase fluctuations and currents in the system behave both spatially and temporally. We discover an unusual current flow in the superconductor after the pulse has subsided, resembling backward waves that normally require special engineering in metamaterials or wave guides. Our results predict a rich behavior of the superconducting order parameter at a microscopic level which is manifested in current textures that can be probed using radiation detection.

cond-mat.supr-con↗

arXiv2026

Real-time detection of critical slowing-down at the superconducting phase transition

We employ optical pump-THz probe spectroscopy to chart the ultrafast superconductivity suppression in NbN over a broad range of excitation fluences. Our measurements uncover a pronounced lengthening of the superconductivity quenching time when the absorbed optical energy is close to the condensation energy of the superconductor, which constitutes a non-equilibrium analog of critical slowing-down on a timescale comparable to that of superconducting fluctuations. Time-dependent Ginzburg-Landau simulations reproduce this behavior and ascribe it to the flattening of the free energy landscape at the dynamical phase transition boundary. Our findings represent a direct observation in real time of slowed-down superconductivity dynamics in proximity to a critical point and open a pathway for investigating out-of-equilibrium critical phenomena with time-resolved THz spectroscopy.

cond-mat.supr-con↗

arXiv2025

Navigating Chiral Spin Architectures in Non-Collinear Antiferromagnetic Thin Films

Noncollinear antiferromagnets offer much promise for antiferromagnetic spintronics and neuromorphic applications with a plethora of functional properties surpassing many competing magnetic systems. Films grown on mismatched substrates may relieve strain by the creation of slip-plane defects that strongly manipulate global physical properties important for application. This work demonstrates that a post growth annealing strategy results in near-defect-free, structurally robust films that reveal the underlying thermal evolution of the magnetic order symmetry. Beyond a critical film thickness, the spin structure transitions between two right-handed irreducible chiral representations via left-handed chiral ordering producing a striking change in the sign and angular dependence of the Anomalous Hall coefficient. The previously established mechanism of spin rotations in the (111) plane cannot fully explain the transition in applied magnetic field, and using a macrospin model this work finds that rotations along the chirality-inverting [1-10] direction are energetically preferable under certain conditions. These observations suggest that both left-handed and right-handed chiral order can be accessed in a single system, opening new routes to engineer devices by controlled switching of magnetic chirality allowing selection of associated functional properties as governed by symmetry.

cond-mat.mtrl-sci↗

arXiv2024

Origin of the laser-induced picosecond spin current across magnetization compensation in ferrimagnetic GdCo

The optical manipulation of magnetism enabled by rare earth-transition metal ferrimagnets holds the promise of ultrafast, energy efficient spintronic technologies. This work investigates laser-induced picosecond spin currents generated by ferrimagnetic GdCo via terahertz emission spectroscopy. A suppression of the THz emission and spin current is observed at magnetization compensation when varying the temperature or alloy composition in the presence of a magnetic field. It is demonstrated that this is due to the formation of domains in the GdCo equilibrium magnetic configuration. Without an applied magnetic field, the picosecond spin current persists at the compensation point. The experimental findings support the model for THz spin current generation based on transport of hot spin-polarized electrons, which is dominated by the Co sublattice at room temperature. Only at low temperature a comparable contribution from Gd is detected but with slower dynamics. Finally, spectral analysis reveals a blueshift of the THz emission related to the formation of magnetic domains close to magnetization compensation.

cond-mat.mes-hall↗

arXiv2024

Ultra-high spin emission from antiferromagnetic FeRh

An antiferromagnet emits spin currents when time-reversal symmetry is broken. This is typically achieved by applying an external magnetic field below and above the spin-flop transition or by optical pumping. In this work we apply optical pump-THz emission spectroscopy to study picosecond spin pumping from metallic FeRh as a function of temperature. Intriguingly we find that in the low-temperature antiferromagnetic phase the laser pulse induces a large and coherent spin pumping, while not crossing into the ferromagnetic phase. With temperature and magnetic field dependent measurements combined with atomistic spin dynamics simulations we show that the antiferromagnetic spin-lattice is destabilised by the combined action of optical pumping and picosecond spin-biasing by the conduction electron population, which results in spin accumulation. We propose that the amplitude of the effect is inherent to the nature of FeRh, particularly the Rh atoms and their high spin susceptibility. We believe that the principles shown here could be used to produce more effective spin current emitters. Our results also corroborate the work of others showing that the magnetic phase transition begins on a very fast picosecond timescale, but this timescale is often hidden by measurements which are confounded by the slower domain dynamics.

cond-mat.mtrl-sci↗

arXiv2023

Helicity-dependent Ultrafast Photocurrents in Weyl Magnet Mn$_3$Sn

We present an optical pump-THz emission study on non-collinear antiferromagnet Mn$_3$Sn. We show that Mn$_3$Sn acts as a source of THz radiation when irradiated by femtosecond laser pulses. The polarity and amplitude of the emitted THz fields can be fully controlled by the polarisation of optical excitation. We explain the THz emission with the photocurrents generated via the photon drag effect by combining various experimental measurements as a function of pump polarisation, magnetic field, and sample orientation with thorough symmetry analysis of response tensors.

cond-mat.mes-hall↗

arXiv2022

The importance of the interface for picosecond spin pumping in antiferromagnet-heavy metal heterostructures

Interfaces between heavy metals (HMs) and antiferromagnetic insulators (AFIs) have recently become highly investigated and debated systems in the effort to create spintronic devices able to function at terahertz frequencies. Such heterostructures have great technological potential because AFIs can generate sub-picosecond spin currents which the HMs can convert into charge signals. In this work we demonstrate an optically induced picosecond spin transfer at the interface between AFIs and Pt using time-resolved THz emission spectroscopy. We select two antiferromagnets in the same family of fluoride cubic perovskites, KCoF3 and KNiF3, whose magnon frequencies at the centre of the Brillouin zone differ by an order of magnitude. By studying their behaviour with temperature we correlate changes in the spin transfer efficiency across the interface to the opening of a gap in the magnon density of states below the Néel temperature. Our observations are reproduced in a model based on the spin exchange between the localized electrons in the antiferromagnet and the free electrons in Pt. These results constitute an important step in the rigorous investigation and understanding of the physics of AFIs/HMs interfaces on the ultrafast timescale.

cond-mat.mes-hall↗

arXiv2021

Boosting spintronics with superconductivity

Spintronics aims to utilize the spin degree of freedom for energy-efficient, non-volatile memory and logic devices. In this research update, we review state-of-the-art developments and new directions in charge- and spin-based memory/logic with a focus on spintronics and the fascinating potential for superconductivity to boost spin transmission via spin-polarized quasiparticles or triplet Cooper pairs.

cond-mat.supr-con↗

arXiv2019

Tunable pure spin supercurrents and the demonstration of a superconducting spin-wave device

Recent ferromagnetic resonance experiments and theory of Pt/Nb/Ni8Fe2 proximity-coupled structures strongly suggest that spin-orbit coupling (SOC) in Pt in conjunction with a magnetic exchange field in Ni8Fe2 are the essential ingredients to generate a pure spin supercurrent channel in Nb. Here, by substituting Pt for a perpendicularly magnetized Pt/Co/Pt spin-sink, we are able to demonstrate the role of SOC, and show that pure spin supercurrent pumping efficiency across Nb is tunable by controlling the magnetization direction of Co. By inserting a Cu spacer with weak SOC between Nb and Pt/(Co/Pt) spin-sink, we also prove that Rashba type SOC is key for forming and transmitting pure spin supercurrents across Nb. Finally, by engineering these properties within a single multilayer structure, we demonstrate a prototype superconducting spin-wave (SW) device in which lateral SW propagation is gateable via the opening or closing of a vertical pure spin supercurrent channel in Nb.

cond-mat.mes-hall↗

arXiv2019

Abrikosov vortex nucleation and its detrimental effect on superconducting spin pumping in Pt/Nb/Ni80Fe20/Nb/Pt proximity structures

We report Abrikosov vortex nucleation in Pt/Nb/Ni80Fe20/Nb/Pt proximity-coupled structures under oblique ferromagnetic resonance (FMR) that turns out to be detrimental to superconducting spin pumping. By measuring an out-of-plane field-angle θH dependence and comparison with Pt-absent control samples, we show that as θH increases, the degree of enhancement (suppression) of spin pumping efficiency in the superconducting state for the Pt-present (Pt-absent) sample diminishes and it reverts to the normal state value at θH = 90°. This can be explained in terms of a substantial out-of-plane component of the resonance field for the Ni80Fe20 layer (with in-plane magnetization anisotropy and high aspect ratio) that approaches the upper critical field of the Nb, turning a large fraction of the singlet superconductor volume into the normal state.

cond-mat.mes-hall↗

arXiv2019

Low-impedance superconducting microwave resonators for strong coupling to small magnetic mode volumes

Recent experiments on strongly coupled microwave and ferromagnetic resonance modes have focused on large volume bulk crystals such as yttrium iron garnet, typically of millimeter-scale dimensions. We extend these experiments to lower volumes of magnetic material by exploiting low-impedance lumped-element microwave resonators. The low impedance equates to a smaller magnetic mode volume, which allows us to couple to a smaller number of spins in the ferromagnet. Compared to previous experiments, we reduce the number of participating spins by two orders of magnitude, while maintaining the strength of the coupling rate. Strongly coupled devices with small volumes of magnetic material may allow the use of spin orbit torques, which require high current densities incompatible with existing structures.

cond-mat.mes-hall↗

arXiv2019

Effect of Meissner screening and trapped magnetic flux on magnetization dynamics in thick Nb/Ni80Fe20/Nb trilayers

We investigate the influence of Meissner screening and trapped magnetic flux on magnetization dynamics for a Ni80Fe20 film sandwiched between two thick Nb layers (100 nm) using broadband (5-20 GHz) ferromagnetic resonance (FMR) spectroscopy. Below the superconducting transition Tc of Nb, significant zero-frequency line broadening (5-6 mT) and DC resonance field shift (50 mT) to a low field are both observed if the Nb thickness is comparable to the London penetration depth of Nb films (>= 100 nm). We attribute the observed peculiar behaviors to the increased incoherent precession near the Ni80Fe20/Nb interface and the effectively focused magnetic flux in the middle Ni80Fe20 caused by strong Meissner screening and (defect-)trapped flux of the thick adjacent Nb layers. This explanation is supported by static magnetic properties of the samples and comparison with FMR data on thick Nb/Ni80Fe20 bilayers. Great care should therefore be taken in the analysis of FMR response in ferromagnetic Josephson structures with thick superconductors, a fundamental property for high-frequency device applications of spin-polarized supercurrents.

cond-mat.mes-hall↗

arXiv2017

Dynamics of skyrmionic states in confined helimagnetic nanostructures

In confined helimagnetic nanostructures, skyrmionic states in the form of incomplete and isolated skyrmion states can emerge as the ground state in absence of both external magnetic field and magnetocrystalline anisotropy. In this work, we study the dynamic properties (resonance frequencies and corresponding eigenmodes) of skyrmionic states in thin film FeGe disk samples. We employ two different methods in finite-element based micromagnetic simulation: eigenvalue and ringdown method. The eigenvalue method allows us to identify all resonance frequencies and corresponding eigenmodes that can exist in the simulated system. However, using a particular experimentally feasible excitation can excite only a limited set of eigenmodes. Because of that, we perform ringdown simulations that resemble the experimental setup using both in-plane and out-of-plane excitations. In addition, we report the nonlinear dependence of resonance frequencies on the external magnetic bias field and disk sample diameter and discuss the possible reversal mode of skyrmionic states. We compare the power spectral densities of incomplete skyrmion and isolated skyrmion states and observe several key differences that can contribute to the experimental identification of the state present in the sample. We measure the FeGe Gilbert damping, and using its value we determine what eigenmodes can be expected to be observed in experiments. Finally, we show that neglecting the demagnetisation energy contribution or ignoring the magnetisation variation in the out-of-film direction - although not changing the eigenmode's magnetisation dynamics significantly - changes their resonance frequencies substantially. Apart from contributing to the understanding of skyrmionic states physics, this systematic work can be used as a guide for the experimental identification of skyrmionic states in confined helimagnetic nanostructures.

cond-mat.mes-hall↗

arXiv2014

Magnonic Charge Pumping via Spin-Orbit Coupling

The interplay between spin, charge, and orbital degrees of freedom has led to the development of spintronic devices like spin-torque oscillators, spin-logic devices, and spin-transfer torque magnetic random-access memories. In this development spin pumping, the process where pure spin-currents are generated from magnetisation precession, has proved to be a powerful method for probing spin physics and magnetisation dynamics. The effect originates from direct conversion of low energy quantised spin-waves in the magnet, known as magnons, into a flow of spins from the precessing magnet to adjacent normal metal leads. The spin-pumping phenomenon represents a convenient way to electrically detect magnetisation dynamics, however, precessing magnets have been limited so far to pump pure spin currents, which require a secondary spin-charge conversion element such as heavy metals with large spin Hall angle or multi-layer layouts to be detectable. Here, we report the experimental observation of charge pumping in which a precessing ferromagnet pumps a charge current, demonstrating direct conversion of magnons into high-frequency currents via the relativistic spin-orbit interaction. The generated electric current, differently from spin currents generated by spin-pumping, can be directly detected without the need of any additional spin to charge conversion mechanism and amplitude and phase information about the relativistic current-driven magnetisation dynamics. The charge-pumping phenomenon is generic and gives a deeper understanding of the recently observed spin-orbit torques, of which it is the reciprocal effect and which currently attract interest for their potential in manipulating magnetic information. Furthermore, charge pumping provides a novel link between magnetism and electricity and may find application in sourcing alternating electric currents.

cond-mat.mes-hall↗

arXiv2009

Bouncing transient currents and SQUID-like voltage in nano devices at half filling

Nanorings asymmetrically connected to wires show different kinds of quantum interference phenomena under sudden excitations and in steady current conditions. Here we contrast the transient current caused by an abrupt bias to the magnetic effects at constant current. A repulsive impurity can cause charge build-up in one of the arms and reverse current spikes. Moreover, it can cause transitions from laminar current flow to vortices, and also change the chirality of the vortex. The magnetic behavior of these devices is also very peculiar. Those nano-circuits which consist of an odd number of atoms behave in a fundamentally different manner compared to those which consist of an even number of atoms. The circuits having an odd number of sites connected to long enough symmetric wires are diamagnetic; they display half-fluxon periodicity induced by many-body symmetry even in the absence of electron-phonon and electron-electron interactions. In principle one can operate a new kind of quantum interference device without superconductors. Since there is no gap and no critical temperature, one predicts qualitatively the same behavior at and above room temperature, although with a reduced current. The circuits with even site numbers, on the other hand, are paramagnetic.

cond-mat.mes-hall↗