arXiv subjects

John Kay Dewhurst

Publications and source records attributed to John Kay Dewhurst.

9 recordsLinked to original sources

arXiv2026

Ultrafast Néel vector switching

We predict ultrafast switching in a chiral anti-ferromagnet that occurs at femtosecond times, nearly 5 orders of magnitude faster than the torque induced nanosecond switching previously observed. The physical mechanism, quite different from that which drives slow switching, involves the creation of massive effective magnetic fields by ultrafast spin current injection. Identifying these fields as key to femtosecond rotation, we establish simple practical rules for their maximisation with wide applicability to all magnetised materials. Employing state-of-the-art time-dependent density-functional theory and using the example of chiral magnet, Mn$_3$Sn, we induce ultrafast rotation enough to drive the switching of magnetic order between the six possible non-collinear ground states. We further demonstrate the possibility of undoing this switching by subsequent injection of oppositely polarized spin current. Our findings place chiral anti-ferromagnets as a materials platform for femtosecond Néel-vector switching, opening a route towards the manipulation of magnetic matter at ultrafast times.

cond-mat.mtrl-sci↗

arXiv2026

Magnetizing altermagnets by ultrafast asymmetric spin dynamics

Laser pulses are known to induce symmetric demagnetization: equal loss of magnetic moments in the identical sublattices of antiferromagnets and ferromagnets at ultrashort timescales. Using time-dependent density functional theory, we show that linearly polarized laser pulses can drive asymmetric demagnetization between otherwise identical sublattices in the $d$-wave compensated altermagnet (AM) RuO$_2$, resulting in a \textit{photo-induced ferrimagnetic state} with a strong net magnetization of $\sim$0.2 $μ_B$ per unit cell. The sign and magnitude of this metastable magnetization are highly controllable by laser polarization. We identify polarization-selective asymmetric optical intersite spin transfer (a-OISTR) as the primary mechanism generating the net moment, followed by asymmetric spin flips (a-SF) that further amplifies it. Both effects originate from the characteristic nodal spin band topology of \textit{d}-wave AMs. Moreover, we demonstrate that this laser-induced magnetization is universal across various $d$-wave AMs, including experimentally confirmed KV$_2$Se$_2$O and RbV$_2$Te$_2$O. We uncover a robust route to light-controlled magnetization in AMs on ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast Saddletronics

Low energy valleys in the band structure of 2d materials represent a potential route to the ultrafast writing of information in quantum matter by laser light, with excited charge at the K or K$^\ast$ valleys representing the fundamental states of 1 and 0. Here we demonstrate that a second electronic feature, the saddle point, is endowed with lightwave control over information states. Linearly polarized light is shown to excite 2 of the 3 inequivalent M point saddles in graphene, generating three possible excited configurations, with which of these are realised determined by the polarization vector direction. We show that saddle excitation is highly robust, with "saddle polarized" states created both in the sub-cycle strong field regime and the long time limit of extended multi-cycle pulses. Our findings, applicable to other members of the graphene family and Xenes such as stanene, point towards a rich and ultrafast light based manipulation of matter based on the saddle point.

cond-mat.mes-hall↗

arXiv2025

Giant moment increase by ultrafast laser light

It is now well established that a few femtosecond laser pulse will induce an ultrafast loss of moment in a magnetic material. Here we show that the opposite effect can also occur: an ultrafast increase in moment. Employing both tight-binding and state-of-the-art time dependent density functional theory we find that laser light tuned to the majority spin conduction band in the 2d magnets CrI$_3$ and CrSBr generates an ultrafast giant moment increase, of up to 33\% in the case of CrI$_3$ (2~$μ_B$). Underpinning this is spin-orbit induced valence band spin texture that, in combination with a strong field light pulse, facilitates an optical spin flip transition involving both intra- and inter-band excitation. Our findings, that establish a general mechanism by which ultrafast light pulses may enhance as well as decrease the magnetic moment, point towards rich possibilities for light control over magnetic matter at femtosecond times.

cond-mat.mtrl-sci↗

arXiv2023

How closely does transient magnetic linear dichroism follow the spin moment?

In highly out-of-equilibrium states of matter, such as those induced by a pump laser, the applicability of well established spectroscopic probes of magnetic order are called into question. Here we address the validity of x-ray absorption techniques in pump laser conditions, focusing on magnetic linear dichroism (MLD), a crucial probe of antiferromagnetic (AFM) order. We directly compute the dynamics of the square of the spin moment and compare to those obtained via the MLD response. For AFM FePd the agreement between these distinct routes to the magnetic moment severely degrades at pulse fluences greater than 1 mJ/cm$^2$, indicating a breakdown of the MLD response as an accurate probe of the transient moment. This contrasts with the MLD for ferromagnetic FePt which reliably tracks the moment for fluences (and absorbed energies) up to an order of magnitude greater than the breakdown threshold for AFM FePd. The underlying microscopic reason for this we find to be increased laser induced excitations out of the $d$-band in AFM FePd, where this increase is made possible by the AFM pseudogap.

cond-mat.mtrl-sci↗

arXiv2023

Verification of ultrafast spin transfer effects in FeNi alloys

The optical intersite spin transfer (OISTR) effect was recently verified in Fe$_{50}$Ni$_{50}$ using magneto-optical Kerr measurements in the extreme ultraviolet range. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe$_{50}$Ni$_{50}$, Fe$_{19}$Ni$_{81}$ and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

cond-mat.mtrl-sci↗

arXiv2023

Unraveling Femtosecond Spin and Charge Dynamics with EUV T-MOKE Spectroscopy

The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV) regime has helped to elucidate some of the key processes that lead to the manipulation of magnetism on ultrafast timescales. However, as we show in this paper, the recently introduced spectrally-resolved analysis of such data can lead to surprising experimental observations, which might cause misinterpretations. Therefore, an extended analysis of the EUV magneto-optics is necessary. Via experimental determination of the dielectric tensor, we find here that the non-equilibrium excitation in an ultrafast magnetization experiment can cause a rotation of the off-diagonal element of the dielectric tensor in the complex plane. In direct consequence, the commonly analyzed magneto-optic asymmetry may show time-dependent behaviour that is not directly connected to the magnetic properties of the sample. We showcase such critical observations for the case of ultrafast magnetization dynamics in Ni, and give guidelines for the future analysis of spectrally-resolved magneto-optical data and its comparison with theory.

cond-mat.mtrl-sci↗

arXiv2023

Partially deorbitalized meta-GGA

Mejia-Rodriguez and Trickey recently proposed a procedure for removing the explicit dependence of meta-GGA exchange-correlation energy functionals $E_{\rm xc}$ on the kinetic energy density $τ$. We present a simple modification to this approach in which the exact Kohn-Sham $τ$ is used as input for $E_{\rm xc}$ but the functional derivative of $τ$ with respect to the density $ρ$, required to calculate the potential term $\int d^3r'\,δE_{\rm xc}/δτ({\bf r}')|_ρ\cdot δτ({\bf r}')/δρ({\bf r})$, is evaluated using an approximate kinetic energy density functional. This ensures that the Kohn-Sham potential is a local multiplicative function as opposed to the non-local potential of a generalized Kohn-Sham approach. Electronic structure codes can be easily modified to use the new method. We validate it by quantifying the accuracy of the predicted lattice parameters, bulk moduli, magnetic moments and cohesive energies of a large set of periodic solids. An unanticipated benefit of this method is to gauge the quality of approximate kinetic energy functionals by checking if the self-consistent solution is indeed at the variational minimum.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic phase diagram of the austenitic Mn-rich Ni-Mn-(In,Sn) Heusler alloys

Heusler compounds have been intensively studied owing to the important technological advancements that they provide in the field of shape memory, thermomagnetic energy conversion and spintronics. Many of their intriguing properties are ultimately governed by their magnetic states and understanding and possibly tuning them is evidently of utmost importance. In this work we examine the \alloys alloys with Density Functional Theory simulations and $^{55}$Mn Nuclear Magnetic Resonance and combine these two methods to carefully describe their ground state magnetic order. In addition, we compare the results obtained with the conventional generalized gradient approximation with the ones of strongly constrained and appropriately normed (SCAN) semilocal functionals for exchange and correlation. Experimental results eventually allow to discriminate between two different scenarios identified by ab initio simulations.

cond-mat.mtrl-sci↗