arXiv subjects

Sam Shallcross

Publications and source records attributed to Sam Shallcross.

15 recordsLinked to original sources

arXiv2026

All optical ultrafast pure spin current in the altermagnet Cr$_2$SO

All-optical generation of pure spin current -- the flow of spin in the absence of a corresponding charge flow -- relies on a symmetry based compensation of valley charge. The 2d $d$-wave altermagnets, ideal spintronics materials due to a very low spin-orbit coupling, possess a magnetic point group and highly anisotropic valley manifolds that would appear to preclude such current compensation, excluding them as materials for the ultrafast generation of pure spin current. Here we show that infra-red valley excitation combined with a THz pulse envelope allows the generation of large and nearly 100\% pure spin currents in the altermagnet Cr$_2$SO. Our approach is based on a valley selection rule coupling linearly polarized light to spin opposite valleys, along with the intrinsic momentum shift that a co-occurring THz pulse imbues a valley spin excitation with. These results thus provide a practical and all-optical route to the generation of pure spin current in $d$-wave 2d altermagnets, opening a route to lightwave control of spin in an environment with very low intrinsic spin mixing.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast ghost Hall states in a 2d altermagnet

Two-dimensional materials that exhibit optically active spin and valley degrees of freedom represent one of the most fascinating -- and potentially most technologically useful -- platforms for the ultrafast interaction of light and matter. Here we show, via the example of Cr$_2$SO, that two dimensional altermagnets host valley states controllable by femtosecond laser light: linearly polarized light pulses excite charge at one of two inequivalent valleys, with which valley charge is excited at determined by the polarization vector direction. This underpins a rich spin and valley physics including: (i) valleytronics $-$ the generation of nearly 100$\%$ spin polarized valley currents, as well as (ii) a "ghost Hall" effect $-$ the ultrafast creation of states in which spin and charge currents are orthogonal without invoking Hall physics. Our findings establish 2d altermagents as a platform providing a new route for the control of spin- and charge currents at ultrafast times.

cond-mat.mes-hall↗

arXiv2025

Valley polarization of graphene via the saddle point

Graphene, and other members of the monolayer Xene family, represent an ideal materials platform for "valleytronics", the control of valley localized charge excitations. The absence of a gap in these semi-metals, however, precludes valley excitation by circularly polarized light pulses, sharply circumscribing the possibility of a lightwave valleytronics in these materials. Here we show that combining a deep ultraviolet linearly polarized light pulse with a THz envelope can induce highly valley polarized states in graphene. This dual frequency lightform operates by (i) the deep ultraviolet pulse activating a selection rule at the M saddle points and (ii) the THz pulse displacing the M point excitation to one of the low-energy K valleys. Employing both tight-binding and state-of-the-art time dependent density functional theory, we show that such a pulse results in a near perfect valley polarized excitation in graphene, thus providing a route via the saddle point to a lightwave valleytronics in the gapless Xene family.

physics.optics↗

arXiv2025

Generation of pure, spin polarized, and unpolarized charge currents at the few cycle limit of circularly polarized light

In certain members of the transition metal dichalcogenide (TMDC) family, laser pulses of oppositely circularly polarized light excite electrons of opposite spin. Here we show that in the few cycle limit such pulses generate not only a spin density excitation, but also a spin current excitation. Employing the example of the TMDC WSe$_2$ we show that pure spin currents, the flow of spin in the absence of net charge flow, 100% spin polarized currents, and charge currents are all accessible and controllable by tuning the amplitude of ~ 5 femtosecond gap tuned light pulses. Underpinning this physics is a symmetry lowering of the valley charge excitation from C3 at long duration to C2 in the few cycle limit, imbuing the excitation with net current. Our results both highlight the emergence of a rich light-spin current coupling at ultrafast times in the TMDC family, as well presenting a route to the all-optical generation of pure spin currents.

cond-mat.mes-hall↗

arXiv2025

Coupled femto-excitons, free carriers and light

Non-equilibrium quantum matter generated by ultrafast laser light opens new pathways in fundamental condensed matter physics, as well as offering rich control possibilities in "tailoring matter by light". Here we explore the coupling between free carriers and excitons mediated by femtosecond scale laser pulses. Employing monolayer WSe$_2$ and an {\it ab-initio} treatment of pump-probe spectroscopy we find that, counter-intuitively, laser light resonant with the exciton can generate massive enhancement of the early time free carrier population. This exhibits complex dynamical correlation to the excitons, with an oscillatory coupling between free carrier population and exciton peak height that persists. Our results both unveil "femto-excitons" as possessing a rich femtosecond dynamics as well as, we argue, allowing tailoring of early time light-matter interaction via laser pulse design to control simultaneously excitonic and free carrier physics at ultrafast times.

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~$\mu_B$). 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↗

arXiv2024

Creation and control of valley currents in graphene by few cycle light pulses

Well established for the visible spectrum gaps of the transition metal dichalcogenide family, valleytronics - the control of valley charge and current by light - is comparatively unexplored for the THz gaps that characterize graphene and topological insulators. Here we show that few cycle pulses of THz light can create and control a 100% valley polarized current in graphene, with light wave control over the current magnitude and direction. The latter is equal to an emergent pulse property of few cycle circularly polarized pulses, the "global" carrier envelope phase. Our findings both highlight the richness of few cycle light pulses in control over quantum matter, and provide a route towards a "THz valleytronics" in meV gapped systems.

cond-mat.mes-hall↗

arXiv2024

Ultrafast all-optical generation of pure spin and valley currents

Pure currents comprise the flow of a two state quantum freedom -- for example the electron spin -- in the absence of charge flow. Radically different from the charge currents that underpin present day electronics, in two dimensional materials possessing additional two state freedoms such as valley index they offer profound possibilities for miniaturization and energy efficiency in a next generation spin- and valley- tronics. Here we demonstrate a robust multi-pump light wave protocol capable of generating both pure spin and valley currents on femtosecond times. The generation time is determined by the 2d material gap, with the creation of pure spin current in WSe2 at 40 fs and pure valley current in bilayer graphene at ~200 fs. Our all-optical approach demands no special material design, requiring only a gapped valley active material, and is thus applicable to a wide range of 2d materials.

cond-mat.mes-hall↗

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↗

arXiv2021

Ab-initio study of ultrafast spin dynamics in Gdx(FeCo){1-x} alloys

Using an ultrashort laser pulse we explore {\it ab-initio} the spin dynamics of Gd$_x$(FeCo)$_{1-x}$ at femtosecond time scales. Optical excitations are found to drive charge from Fe majority $d$-states to the unoccupied Gd $f$-minority states, with $f$-electron character excited occupation lagging behind that of the $d$-electron character, leading to substantial demagnetisation of both species while leaving the global moment almost unchanged. For $x > 0.33$ this results in the creation of an ultrafast ferromagnetic (FM) transient by the end of the laser pulse, with the Gd demagnetization rate slower than that of Fe. For all concentrations the Gd moments begin to rotate from their ground state orientations developing in-plane moments of between 0.2-0.5~$\mu_B$. Thus, the ultrafast spin dynamics of the material captures three important ingredients of the all optical switching that occurs at much later (picosecond) times: (i) the development of a FM transient, (ii) the different rates of demagnetisation of Fe and Gd and, (iii), the breaking of the colinear symmetry of the ground state. Furthermore, several predictions are made about the behaviour of Fe-Gd alloys that can be experimentally tested and can lead to a spin-filtering device.

cond-mat.mtrl-sci↗

arXiv2020

Uncovering the role of the density of states in controlling ultrafast spin dynamics

At the ultrafast limit of optical spin manipulation is the theoretically predicted phenomena of optical intersite spin transfer (OISTR), in which laser induced charge transfer between the sites of a multi-component material leads to control over magnetic order. A key prediction of this theory is that the demagnetization efficiency is determined by the availability of unoccupied states for intersite charge transfer. Employing state-of-the-art magneto-optical Kerr effect measurements with femtosecond time resolution, we probe this prediction via a systematic comparison of the ultrafast magnetic response between the 3d ferromagnets, Fe, Co, and Ni, and their respective Pt-based alloys and multilayers. We find that (i) the demagnetization efficiency in the elemental magnets increases monotonically from Fe, via Co to Ni and, (ii), that the gain in demagnetization efficiency of the multi-component system over the pure element counterpart scales with the number of empty 3d minority states, exactly as predicted by the OISTR effect. We support these experimental findings with ab initio time-dependent density functional theory calculations that we show to capture the experimental trends very well.

cond-mat.mtrl-sci↗

arXiv2019

Perfect and controllable nesting in the small angle twist bilayer graphene

Parallel ("nested") regions of a Fermi surface (FS) drive instabilities of the electron fluid, for example the spin density wave in elemental chromium. In one-dimensional materials, the FS is trivially fully nested (a single nesting vector connects two "Fermi dots"), while in higher dimensions only a fraction of the FS consists of parallel sheets. We demonstrate that the tiny angle regime of twist bilayer graphene (TBLG) possess a phase, accessible by interlayer bias, in which the FS consists entirely of nestable "Fermi lines": the first example of a completely nested FS in a 2d material. This nested phase is found both in the ideal as well as relaxed structure of the twist bilayer. We demonstrate excellent agreement with recent STM images of topological states in this material and elucidate the connection between these and the underlying Fermiology. We show that the geometry of the "Fermi lines" network is controllable by the strength of the applied interlayer bias, and thus that TBLG offers unprecedented access to the physics of FS nesting in 2d materials.

cond-mat.mtrl-sci↗

arXiv2016

Anomalous Dirac point transport due to extended defects in bilayer graphene

Charge transport at the Dirac point in bilayer graphene exhibits two dramatically different transport states, insulating and metallic, that occur in apparently otherwise indistinguishable experimental samples. We demonstrate that the existence of these two transport states has its origin in an interplay between evanescent modes, that dominate charge transport near the Dirac point, and disordered configurations of extended defects in the form of partial dislocations. In a large ensemble of bilayer systems with randomly positioned partial dislocations, the conductivity distribution $P(\sigma)$ is found to be strongly peaked at both the insulating and metallic limits. We argue that this distribution form, that occurs only at the Dirac point, lies at the heart of the observation of both metallic and insulating states in bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2005

Pt steps decorated by 3d nanowires calculated via an order-N method

We present the magnetization energy and magnetic moments of 3d monowires aligned along the step edges of Pt (533) and (322) vicinal substrates. We employ an electronic structure method which scales linearly with the size of the system. This allows us to treat with ease both ferromagnetic and antiferromagnetic solutions, and in principle the method should allow for the evaluation of more complex systems, such as wires deposited on rough step edges. We find that Co, Fe, and Mn are almost perfect Heisenberg systems, with almost no change in the moment between antiferromagnetic and ferromagnetic solutions. A large polarisation of the Pt substrate is also observed. Our results are in close agreement w ith recent experimental results obtained for the case of a Co monowire. As expected the trend is for the late d series to be stabalised by ferromagnetic long range order and the middle of the d series by antiferromagnetic order. Since our method is only \emph{locally} self consistent we discuss as some length the convergence of the results with respect to the size of the region which is treated self-consistently.

cond-mat.mtrl-sci↗