arXiv subjects

Rubén M. Otxoa

Publications and source records attributed to Rubén M. Otxoa.

17 recordsLinked to original sources

arXiv2026

Temperature dependence of spin-model parameters in ferrimagnets

We compute the temperature dependence of effective spin-model parameters in two-sublattice ferrimagnets with antiparallel and inequivalent sublattice magnetizations using Green's-function theory. The effective intra- and intersublattice exchange interactions, together with the sublattice anisotropies, exhibit distinct thermal renormalizations arising from the different magnetic interactions and correlations of the two sublattices. Although these effective interaction parameters decrease monotonically with increasing temperature, their unequal renormalization produces a strongly nonmonotonic and polarization-dependent long-wavelength spin-wave response. In particular, the spin-wave exchange stiffness exhibits a strongly branch-dependent and nonmonotonic temperature dependence. One polarization develops an enhanced spin-wave stiffness in the vicinity of the angular momentum compensation region despite the monotonic thermal reduction of the underlying exchange interactions. The Green's-function predictions are in quantitative agreement with atomistic spin-dynamics simulations, which provide a numerical benchmark including thermally induced spin correlations within the underlying atomistic spin model. These results establish a direct connection between temperature-dependent atomistic interactions, long-wavelength spin-wave stiffness, and the effective parameters required for finite-temperature micromagnetic and multiscale modeling of ferrimagnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Moving Analogue Horizons in Stationary Ferroelectrics

We show that a traveling modulation of the polarization-gradient stiffness in a ferroelectric material induces an effective flow in its collective polarization dynamics. Within a controlled local approximation, small polarization fluctuations, or ferrons, obey a massive Klein-Gordon equation with flow. Unlike conventional analogue-gravity platforms, the effective flow originates from the modulation of the material parameters rather than from the physical transport of the medium. This mechanism enables mobile analogue horizons separating sub-ferronic and super-ferronic regions, the latter supporting negative-norm antiferron modes. Their coupling to positive-norm ferrons gives rise to superradiant-like amplification, while the effective mass gap can be tuned independently through an external electric field. Ferroelectric systems therefore provide a novel and experimentally controllable platform for analogue gravity with massive scalar excitations.

cond-mat.mes-hall↗

arXiv2026

Universal theory of domain-wall width in multi-sublattice Heisenberg magnets

We propose a universal expression for the domain-wall width in generic multi-sublattice Heisenberg magnets, applicable to ferro-, antiferro-, and ferrimagnetic orders. The result follows from an exact connection between the domain-wall profile and the long-wavelength spin-wave dispersion, yielding a unified framework for describing magnetic textures across distinct ordering types. The predictions show excellent quantitative agreement with large-scale atomistic spin dynamics simulations over a broad range of exchange and anisotropy values and spin multi-sublattice structures, including three-dimensional rock-salt-type magnets and two-dimensional honeycomb and kagome ferromagnets. Moreover, we establish a general microscopic foundation for the temperature dependence of the domain-wall width. Our approach offers a powerful tool for understanding domain-wall profiles in complex spin systems.

cond-mat.mtrl-sci↗

arXiv2026

Hardware-Tailored Resource Estimation for Magic-State Distillation on Silicon Spin Qubits

We present a resource analysis for generating high-fidelity logical magic states on silicon spin-qubit platforms. We consider a range of architectures, including a shuttling-based SpinBus design, a dense nearest-neighbor layout, and a hybrid scheme with shuttling-connected patches. We compare surface, color, and biased error-correcting codes, and analyze the $5\to1$ and $15\to1$ magic-state distillation protocols. Our approach combines bottom-up and top-down methodologies. We construct a hardware-level noise model based on a silicon-processor Hamiltonian with realistic parameters and $1/f$ non-Markovian noise, enabling estimation of physical resources required to reach target logical error rates. These results are propagated to system-level overheads for applications including spin dynamics, integer factorization, and quantum chemistry. Conversely, we fix target logical fidelities and derive corresponding constraints on hardware performance. Our framework enables systematic evaluation of resource-reduction strategies. We find that optimized control pulses reduce magic-state distillation overhead by 42\% compared to standard gate implementations. In addition, silicon-tailored biased error-correcting codes achieve an approximately threefold reduction in physical footprint relative to the surface code, even without physical-bias-preserving operations.

quant-ph↗

arXiv2026

Exact Solution for Current-Driven Domain-Wall Dynamics Beyond Lorentz Contraction in Antiferromagnets with Dzyaloshinskii-Moriya Interaction

We study current-driven domain-wall (DW) dynamics in antiferromagnets (AFMs) with Dzyaloshinskii-Moriya interaction (DMI). We obtain an exact analytical solution for spiral DW dynamics, applicable to both head-to-head DWs under bulk DMI and up-down DWs under interfacial DMI when the magnetic easy axis is aligned with the DMI vector. For the latter case experimentally relevant to synthetic AFMs with in-plane anisotropy, the solution predicts a constant DW velocity driven by nonadiabatic spin-transfer torque together with a steady rotation of the DW tilt angle induced by damping-like spin-orbit torque. Remarkably, the DW width shows unconventional current dependence, either pure elongation or contraction followed by elongation depending on damping and torque parameters, in sharp contrast to the Lorentz-type contraction known for antiferromagnetic (AF) DWs without DMI. These results provide an exact description of current-driven AF-DW dynamics and suggest experimentally accessible signatures of DMI-modified DW dynamics in synthetic AFMs.

cond-mat.mes-hall↗

arXiv2026

Impact of leakage on the dynamics of a ST$_0$ qubit implemented in a Double Quantum Dot device

Spin qubits in quantum dots are a promising technology for quantum computing due to their fast response time and long coherence times. An electromagnetic pulse is applied to the system for a specific duration to perform a desired rotation. To avoid decoherence, the amplitude and gate time must be highly accurate. In this work, we aim to study the impact of leakage during the gate time evolution of a spin qubit encoded in a double quantum dot device. We prove that, in the weak interaction regime, leakage introduces a shift in the phase of the time evolution operator, causing over- or under-rotations. Indeed, controlling the leakage terms is useful for adjusting the time needed to perform a quantum computation and increasing the coherence time of the readout process. This is crucial for running fault-tolerant algorithms and is beneficial for Quantum Error Mitigation techniques.

quant-ph↗

arXiv2025

Sub-GHz Breathing Dynamics of Magnetic Hopfions

Magnetic hopfions are three-dimensional topological solitons whose static stability has recently been confirmed in experiments, yet their dynamical modes remain largely unexplored. Here we combine micromagnetic simulations and analytical modelling to characterise the fundamental breathing excitation of hopfions. We show that the breathing mode corresponds to a coherent oscillation of both the hopfion core diameter and the shell width, while preserving the topological charge. An analytical domain-wall interaction model explains the weak field dependence of the shell thickness and yields a closed-form expression for the restoring stiffness. From this curvature and the collective-coordinate inertia, we derive an estimated breathing frequency in excellent agreement with micromagnetic spectra. The ability to capture the hopfion dynamics quantitatively from material constants highlights a direct route to experimental detection by ferromagnetic resonance or Brillouin light scattering, and establishes a framework for frequency-encoded control in reconfigurable spintronic devices.

cond-mat.mes-hall↗

arXiv2025

Skyrmion Hall effect and shape deformation of current-driven bilayer skyrmions in synthetic antiferromagnets

The commonly believed absence of skyrmion Hall effect for topologically trivial magnetic skyrmions is reconsidered for bilayer skyrmions in synthetic antiferromagnets driven by spin-transfer and spin-orbit torques. Using a general Lagrangian formalism, we show that Bloch-type bilayer skyrmions acquire a finite Hall angle when driven by spin-orbit torque, while Néel-type skyrmions do not, in agreement with micromagnetic simulations. Both types of skyrmions exhibit current-induced elliptical deformation with minor and major axes aligned longitudinally and transversely to their velocity, respectively. A linear relation between velocity and longitudinal radius is derived with a coefficient proportional to the strength of spin-orbit torque. These effects are critical for antiferromagnetic skyrmion-based applications such as skyrmion racetrack memory. The Lagrange equations also reproduce the linear Hall angle-helicity relation reported by Msiska et al., Phys. Rev. Appl. 17, 064015 (2022). An intuitive explanation of the skyrmion Hall effect for arbitrary helicity based on the antiferromagnetic exchange torque is also provided.

cond-mat.mes-hall↗

arXiv2025

SpinHex: A low-crosstalk, spin-qubit architecture based on multi-electron couplers

Semiconductor spin qubits are an attractive quantum computing platform that offers long qubit coherence times and compatibility with existing semiconductor fabrication technology for scale up. Here, we propose a spin-qubit architecture based on spinless multielectron quantum dots that act as low-crosstalk couplers between a two-dimensional arrangement of spin-qubits in a hexagonal lattice. The multielectron couplers are controlled by voltage signals, which mediate fast Heisenberg exchange and thus enable coherent multi-qubit operations. For the proposed architecture, we discuss the implementation of the rotated XZZX surface code and numerically study its performance for a circuit-level noise model. We predict a threshold of $0.18\%$ for the error rate of the entangling gates. We further evaluate the scalability of the proposed architecture and predict the need for $4480$ physical qubits per logical qubit with logical error rates of $10^{-12}$ considering entangling gate fidelities of $99.99\%$, resulting in a chip size of $2.6$cm$^2$ to host $10,000$ logical qubits.

quant-ph↗

arXiv2024

Efficient motion of 90^{\circ} domain walls in Mn_{2}Au via pure optical torques

Discovering alternative ways to drive domain wall (DW) dynamics is crucial for advancing spintronic applications. Here we demonstrate via atomistic spin dynamics simulations that optical torques can efficiently drive 90^{\circ} DWs in the Mn2Au antiferromagnet but their spatial symmetry forbids the motion of 180^{\circ} walls. In the steady-state regime, the kinematics display special relativity signatures accessed for low laser intensities. At velocities higher than the magnonic limit, the DW enters a proliferation regime in which part of its relativistic energy is invested into the nucleation of novel magnetic textures. Our investigation contributes towards the fundamental understanding of opto-magnetic effects, supporting the development of next generation, all-optically controlled antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2024

Predicted Multiple Walker Breakdowns for Current-Driven Domain-Wall Motion in Antiferromagnets

We theoretically discover possible emergence of reentrant Walker breakdowns for current-driven domain walls in layered antiferromagnets in striking contrast to the unique Walker breakdown in ferromagnets. We reveal that the Lorentz contraction of domain-wall width in antiferromagnets gives rise to nonlinear current-dependence of the wall velocity and the predicted multiple Walker breakdowns. The dominant efficiency of the current-induced staggered spin-orbit torque over the spin-transfer torque to drive the domain-wall motion is also demonstrated. These findings are expected to be observed in synthetic antiferromagnets experimentally and provide an important contribution to the growing research field of antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2023

Generalised form of the magnetic anisotropy field in micromagnetic and atomistic spin models

We present a general approach to the derivation of the effective anisotropy field which determines the dynamical behaviour of magnetic spins according to the Landau-Lifshitz-Gilbert equation. The approach is based on the gradient in spherical polar coordinates with the final results being expressed in Cartesian coordinates as usually applied in atomistic and micromagnetic model calculations. The approach is generally valid for all orders of anisotropies including higher order combinations of azimuthal and rotational anisotropies often found in functional magnetic materials such as permanent magnets and an emerging class of antiferromagnetic materials with applications in spintronics. Anisotropies are represented in terms of spherical harmonics which have the important property of rational temperature scaling. Effective field vectors are given for anisotropies up to sixth order, presenting a unified framework for implementing higher order magnetic anisotropies in numerical simulations.

cond-mat.mtrl-sci↗

arXiv2020

Steady one-dimensional domain wall motion in biaxial ferromagnets: mapping of the Landau-Lifshitz equation to the sine-Gordon equation

Motivated by the difference between the dynamics of magnetization textures in ferromagnets and antiferromagnets, the Landau-Lifshitz equation of motion is explored. A typical one-dimensional domain wall in a bulk ferromagnet with biaxial magnetic anisotropy is considered. In the framework of Walker-type of solutions of steady-state ferromagnetic domain wall motion, the reduction of the non-linear Landau-Lifshitz equation to a Lorentz-invariant sine-Gordon equation typical for antiferromagnets is formally possible for velocities lower than a critical velocity of the topological soliton. The velocity dependence of the domain wall energy and the domain wall width are expressed in the relativistic-like form in the limit of large ratio of the easy-plane/easy-axis anisotropy constants. It is shown that the mapping of the Landau-Lifshitz equation of motion to the sine-Gordon equation can be performed only by going beyond the steady-motion Walker-type of solutions.

cond-mat.mtrl-sci↗

arXiv2020

Walker-like Domain Wall breakdown in layered Antiferromagnets driven by staggered spin-orbit fields

Within linear continuum theory, no magnetic texture can propagate faster than the maximum group velocity of its spin waves. Here we report a transient regime due to the appearance of additional antiferromagnetic textures that breaks the Lorentz translational invariance of the magnetic system by atomistic spin dynamics simulations. This dynamical regime is akin to domain wall Walker-breakdown in ferromagnets and involves the nucleation of an antiferromagnetic domain wall pair. Subsequently, one of the nucleated 180$^{\circ}$ domain wall creates with the original domain wall a 360$^{\circ}$ spin-rotation which remains static even under the action of the spin-orbit field. The other 180$^{\circ}$ domain wall becomes accelerated to super-magnonic speeds. Under large spin-orbit fields, multiple domain wall generation and recombination is obtained which may explain the recently experimentally observed current pulse induce shattering of large domain structures into small fragmented domains and the subsequent slow recreation of large-scale domain formation prior current pulse.

cond-mat.mes-hall↗

arXiv2019

A quantum interference capacitor based on double-passage Landau-Zener-Stückelberg-Majorana interferometry

The implementation of quantum technologies in electronics leads naturally to the concept of coherent single-electron circuits, in which a single charge is used coherently to provide enhanced performance. In this work, we propose a coherent single-electron device that operates as an electrically-tunable capacitor. This system exhibits a sinusoidal dependence of the capacitance with voltage, in which the amplitude of the capacitance changes and the voltage period can be tuned by electric means. The device concept is based on double-passage Landau-Zener-Stückelberg-Majorana interferometry of a coupled two-level system that is further tunnel-coupled to an electron reservoir. We test this model experimentally by performing Landau-Zener-Stückelberg-Majorana interferometry in a single-electron double quantum dot coupled to an electron reservoir and show that the voltage period of the capacitance oscillations is directly proportional to the excitation frequency and that the amplitude of the oscillations depends on the dynamical parameters of the system: intrinsic relaxation and coherence time, as well as the tunneling rate to the reservoir. Our work opens up an opportunity to use the non-linear capacitance of double quantum dots to obtain enhanced device functionalities.

quant-ph↗

arXiv2019

Giant localised spin-Peltier effect due to ultrafast domainwalls motion in antiferromagnetic metals

Spin thermo-electric phenomena have attracted wide attention recently, e.g. the Spin Peltier effect (SPE) heat generation by magnonic spin currents. Here we find that the Spin Peltier ef-fect also manifests as a heat wave accompanying fast moving magnetic textures. High speed andextreme magnetic excitation localisation are paramount for efficient transfer of energy from thespin-degrees of freedom to electrons and lattice. While satisfying both conditions is subject to se-vere restrictions in ferromagnets, we find that domain walls in antiferomagnets can overcome theselimitations due to their potential ultrahigh mobility and ultra-small widths originating from the rel-ativistic contraction. To illustrate our findings, we show that electric current driven domain wallmotion in the antiferromagnetic metal Mn2Au can carry a localised heat wave with the maximumamplitude up to 1 K . Since domain walls are well localised nanoscale magnetic objects, this effecthas the potential for nanoscale heating sensing and functionalities.

cond-mat.mes-hall↗

arXiv2015

Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators

We study the magnetization dynamics in a nanocontact magnetic vortex oscillators as function of temperature. Low temperature experiments reveal that the dynamics at low and high currents differ qualitatively. At low currents, we excite a temperature independent standard oscillation mode, consisting in the gyrotropic motion of a free layer vortex about the nanocontact. Above a critical current, a sudden jump of the frequency is observed, concomitant with a substantial increase of the frequency versus current slope factor. Using micromagnetic simulation and analytical modeling, we associate this new regime to the creation of a vortex-antivortex pair in the pinned layer of the spin valve. The vortex-antivortex distance depends on the Oersted field which favors a separation, and on the exchange bias field, which favors pair merging. The pair in the pinned layer provides an additional spin torque altering the dynamics of the free layer vortex, which can be quantitatively accounted for by an analytical model.

cond-mat.mtrl-sci↗