arXiv subjects

Allan S. Johnson

Publications and source records attributed to Allan S. Johnson.

At least 19 recordsLinked to original sources

arXiv2026

Thermodynamics of the ultrafast phase transition of vanadium dioxide

Ultrafast photoexcitation is an emerging route to selective control of phase transitions. However, it is difficult to determine which modes govern the transformation and how effectively they are targeted by photoexcitation. This is exemplified in vanadium dioxide, which transitions from a monoclinic insulator to a rutile metal upon heating or photoexcitation. There is a long-standing debate about whether this transition is electronically or structurally driven and whether the structural component is coherent, driven by a single structural mode or thermal in nature. In this work, we develop a simple thermodynamic framework based on temperature-dependent ultrafast pump-probe measurements and contrast it to microscopic-detail-free modelling to identify the driving mechanism of the transition, revealing that population of the full thermal phonon spectrum, especially high-frequency oxygen modes, is necessary to stabilize the metallic phase. Our approach can straightforwardly be applied to determine the nature of other photoinduced phase transitions without the need for complex multi-messenger experiments and can guide new control strategies, even for incoherent transitions.

cond-mat.str-el↗

arXiv2025

Non-linear X-ray Coherent Diffractive Imaging

The advent of nonlinear X-ray processes like sum-frequency generation and four-wave mixing raises the possibility of non-linear X-ray imaging, combining the high-resolution and elemental specificity of X-ray imaging with the state selectivity and sensitivity of non-linear optical imaging. While scanning imaging methods may be feasible, for linear X-ray processes coherent diffractive imaging has emerged as a key approach, enabling lensless reconstruction of nanoscale structure and dynamics with high spatial and temporal resolution. In this work, we propose a coherent diffractive approach to imaging using X-ray nonlinear processes, introducing an analysis method to isolate the nonlinear component from the overall diffraction pattern by leveraging the property of mutual incoherence between different wavelengths. For examples such as sum-frequency generation in ferroelectrics, this method reveals both domain structure and orientation through the retrieved amplitude and phase of the nonlinear signal. We discuss the feasibility of the proposed method in the presence of experimental noise, most relevantly shot noise. This analysis method is applicable in both static and dynamic imaging, offering a pathway beyond traditional spectroscopy toward XUV/X-ray coherent imaging of spatio-temporal dynamics in quantum materials and biological systems.

physics.optics↗

arXiv2025

Electron coherent phonon coupling in Pr$_{0.5}$Ca$_{1.5}$MnO$_4$ measured with ultrafast broadband spectroscopy

Photoexcitation of single-layered La$_{0.5}$Mn$_{1.5}$MnO$_4$ has played a key role in understanding orbital ordering and non-thermal states in the manganites. However, while orbital ordering in La$_{0.5}$Sr$_{1.5}$MnO$_4$ breaks the in-plane C$_4$ symmetry, many layered manganites show much more complex phase diagrams in which orbital ordering emerges from an already symmetry-broken high-temperature phase and also exhibit additional low-temperature phases. In this work, we examine the role of these phases in relation to orbital ordering in the single-layered manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$ with a combination of optical reflection anisotropy and ultrafast broadband pump-probe spectroscopy. We find that the reflection anisotropy, measured in equilibrium, is strongly sensitive to charge and orbital-ordering transition only. However, the ultrafast response, measuring the non-equilibrium state is sensitive to all phases. In particular, we deduce that coherent phonons modulate unoccupied electronic states that are sensitive to the different phases of the material. This gives rise to a non-linear scaling of the phonon signal with pump fluence at specific probe wavelengths.

cond-mat.str-el↗

arXiv2024

Signature of attochemical quantum interference upon ionization and excitation of an electronic wavepacket in fluoro-benzene

Ultrashort pulses can excite or ionize molecules and populate coherent electronic wavepackets, inducing complex dynamics. In this work, we simulate the coupled electron-nuclear dynamics upon ionization to different electronic wavepackets of (deuterated) benzene and fluoro-benzene molecules, quantum mechanically and in full dimensionality. In fluoro-benzene, the calculations unravel both inter-state and intra-state quantum interferences that leave clear signatures of attochemistry and charge-directed dynamics in the shape of the autocorrelation function. The latter are in agreement with experimental high harmonic spectroscopy measurements of benzenes and fluoro-benzene.

physics.chem-ph↗

arXiv2024

Chirped pulse control over the melting of superconductors

Strong field terahertz pulses are increasingly used to excite and control quantum materials at the ultrafast timescale. They have found widespread application by enabling direct addressing of the superconducting gap or Josephson resonances and are essential in Higgs spectroscopy. Large non-linear optical signals can be induced by the strong coupling of the THz and superconducting degrees of freedom. However, far less attention has been paid to the strong bi-directional coupling between field and material this implies. Here, we use the framework of the time-dependent Ginzburg-Landau equations to study the full field and material evolution of a superconductor driven by strong field terahertz pulses. We find that at high field strengths, the backreaction of the superconductor induces large changes to the driving pulse, which in turn leads to a runaway melting of the superconducting condensate. This results in a surprisingly large sensitivity to the initial driving pulse chirp, enabling these purely dynamical changes to result in order of magnitude different levels of melting. We also find large-scale spectral shifting of the driving pulse to occur in just a few hundred nanometers of propagation through a superconductor. We attribute these effects to an inverse plasma redshift, in which the driving field breaks Cooper pairs and decreases the free-electron mobility, analogous to reducing the density of a plasma.

cond-mat.supr-con↗

arXiv2024

Ultrafast lattice disordering can be accelerated by electronic collisional forces

In the prevalent picture of ultrafast structural phase transitions, the atomic motion occurs in a slowly varying potential energy surface determined adiabatically by the fast electrons. However, this ignores non-conservative forces caused by electron-lattice collisions, which can significantly influence atomic motion. Most ultrafast techniques only probe the average structure and are less sensitive to random displacements, and therefore do not detect the role played by non-conservative forces in phase transitions. Here we show that the lattice dynamics of the prototypical insulator-to-metal transition of VO2 cannot be described by a potential energy alone. We use the sample temperature to control the preexisting lattice disorder before ultrafast photoexcitation across the phase transition and our ultrafast diffuse scattering experiments show that the fluctuations characteristic of the rutile metal develop equally fast (120 fs) at initial temperatures of 100 K and 300 K. This indicates that additional non-conservative forces are responsible for the increased lattice disorder. These results highlight the need for more sophisticated descriptions of ultrafast phenomena beyond the Born-Oppenheimer approximation as well as ultrafast probes of spatial fluctuations beyond the average unit cell measured by diffraction.

cond-mat.str-el↗

arXiv2024

Coherent X-ray Imaging of Stochastic Dynamics

Condensed phase systems often exhibit a mixture of deterministic and stochastic dynamics at the nanoscale which are essential to understanding their function, but can be challenging to study directly using conventional imaging methods. Coherent X-ray imaging has emerged as a powerful tool for studying both nanoscale structures and dynamics in condensed phase systems, including stochastic dynamics, but the requirement to obtain single-shot images in order to obtain freeze-frame images of the stochastic dynamics means the X-ray fluxes used must be very high, potentially destroying the samples. This prevents coherent imaging from being applied to complex systems like tracking the motion of charge carriers or domain fluctuations in quantum materials. Here we show that, by leveraging the coherence intrinsic to these methods, we can separate out the stochastic and deterministic contributions to a coherent X-ray scattering pattern, returning real space images of the deterministic contributions and the momentum spectrum of the stochastic contributions. We further show that, for several typical and important classes of fluctuations, we can return real space images of the mean fluctuations. We demonstrate this approach by numerically simulating the imaging of stochastic polaron separation following photoexcitation and by recovering the spectral properties of fluctuating domain walls. Our versatile approach will enable the direct recovery of the spatial, spectral and temporal properties of stochastic material dynamics in a wide variety of systems currently unobtainable with existing methods.

physics.optics↗

arXiv2024

Decoupled few-femtosecond phase transitions in vanadium dioxide

The nature of the insulator-to-metal phase transition in vanadium dioxide (VO2) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily driven by the electronic or structural degree of freedom, but measurements to date have been stymied by their sensitivity to only one of these components and/or their limited temporal resolution. Here we use ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the electronic and structural phase transitions in VO2 at their fundamental time scales. We find that the system transforms into a bad-metallic phase within 10 fs after photoexcitation, but requires another 100 fs to complete the transition, during which we observe electronic oscillations and a partial re-opening of the bandgap, signalling a transient semi-metallic state. Comparisons with tensor-network simulations and density-functional theory calculations show these features originate from oscillations around the equilibrium high-symmetry atomic positions during an unprecedentedly fast structural transition, in which the vanadium dimers separate and untwist with two different timescales. Our results resolve the complete structural and electronic nature of the light-induced phase transition in VO2 and establish ultra-broadband few-femtosecond spectroscopy as a powerful new tool for studying quantum materials out of equilibrium.

cond-mat.str-el↗

arXiv2024

Light-induced phase transitions in vanadium dioxide: a tensor network study

Nonequilibrium phase transitions driven by light pulses represent a rapidly developing field in condensed matter physics. As one of the archetypal strongly correlated materials, vanadium dioxide (VO2) undergoes a structural phase transition (SPT) from a monoclinic (M1) to rutile (R) structure and an insulator-to-metal transition (IMT) either when heated above 340 K or when excited by an ultrafast laser pulse. Here, we present a tensor network study of the light-induced phase transitions in VO2 based on a quasi-one-dimensional model with all the important ingredients -- multi-orbital character, electron-lattice coupling, and electron-electron correlations -- being included. We show that this model qualitatively captures the equilibrium properties of VO2 by calculating the ground state phase diagram and finite-temperature phase transitions. A hybrid quantum-classical tensor-network method is used to simulate the dynamics following photoexcitation. We find that the structure can transform faster than the harmonic phonon modes of M1 phase, suggesting lattice nonlinearity is key in the SPT. We also find separate timescales for the evolution of dimerization and tilt distortions in the lattice dynamics, as well as the loss and subsequent partial restoration behavior of the displacements, which can provide an explanation for the complex dynamics observed in recent experiments [C. Brahms et al., arXiv:XXXX.XXXXX]. Moreover, decoupled SPT and IMT dynamics are observed in the numerical simulations: while the initial M1 structure transforms to the R one in tens of femtoseconds, the IMT occurs quasi-instantaneously, consistent with recent experimental findings. Our theoretical studies provide insight into the light-induced phase transitions of VO2, revealing unexpected non-monotonic transformation pathways and paving the way for future studies of non-thermal phase transformations.

cond-mat.str-el↗

arXiv2023

Determination and correction of spectral phase from principal component analysis of coherent phonons

Measuring the spectral phase of a pulse is key for performing wavelength resolved ultrafast measurements in the few femtosecond regime. However, accurate measurements in real experimental conditions can be challenging. We show that the reflectivity change induced by coherent phonons in a quantum material can be used to infer the spectral phase of an optical probe pulse with few-femtosecond accuracy.

physics.optics↗

arXiv2023

All-optical seeding of a light-induced phase transition with correlated disorder

Ultrafast manipulation of vibrational coherence is an emergent route to control the structure of solids. However, this strategy can only induce long-range correlations and cannot modify atomic structure locally, which is required in many technologically-relevant phase transitions. Here, we demonstrate that ultrafast lasers can generate incoherent structural fluctuations which are more efficient for material control than coherent vibrations, extending optical control to a wider range of materials. We observe that local, non-equilibrium lattice distortions generated by a weak laser pulse reduce the energy barrier to switch between insulating and metallic states in vanadium dioxide by 6%. Seeding inhomogeneous structural-fluctuations presents an alternative, more energy efficient, route for controlling materials that may be applicable to all solids, including those used in data and energy storage devices.

cond-mat.str-el↗

arXiv2023

Ultrafast X-ray imaging of the light-induced phase transition in VO2

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

cond-mat.str-el↗

arXiv2022

Non-thermal breaking of magnetic order via photo-generated spin defects

In Mott insulators the evolution of antiferromagnetic order to superconducting or charge-density-wave-like states upon chemical doping underpins the control of quantum phases. Photo-doping can induce similar transitions on the ultrafast timescale, however the response of the spin system has remained elusive. Here, we use 4D-ultrafast optical spectroscopy to extract quantitative magnetic dynamics in the spin-orbit coupled Mott insulator Sr3Ir2O7. We demonstrate that light can non-thermally melt long-range spin order. At low fluences magnetic order recovers within 1 ps despite demagnetization of roughly 50%. However, high fluences induce a crossover to a long-lived demagnetized state without increasing the lattice temperature. We show that the generation of photo-induced spin defects enables a mechanism that stabilizes the demagnetized state which could help expose new transient phases.

cond-mat.str-el↗

arXiv2021

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

Spontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La0.5Sr1.5MnO4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

cond-mat.str-el↗

arXiv2021

Fermionic Chern insulator from twisted light with linear polarization

The breaking of time-reversal symmetry is a crucial ingredient to topological bands. It can occur intrisically in materials with magnetic order, or be induced by external fields, such as magnetic fields in quantum Hall systems, or circularly polarized light fields in Floquet Chern insulators. Apart from polarization, photons can carry another degree of freedom, orbital angular momentum, through which time-reversal symmetry can be broken. In this Letter, we pose the question whether this property allows for inducing topological bands via a linearly polarized but twisted light beam. To this end, we study a graphene-like model of electrons on a honeycomb lattice interacting with a twisted light field. To identify topological behavior of the electrons, we calculate their local markers of Chern number, and monitor the presence of in-gap edge states. Our results are shown to be fully analogous to the behavior found in paradigmatic models for static and driven Chern insulators, and realizing the state is experimentally straightforward. With this, our work establishes a new mechanism for generating Fermionic topological phases of matter that can harness the central phase singularity of an optical vortex beam.

cond-mat.mes-hall↗

arXiv2020

Does VO$_2$ Host a Transient Monoclinic Metallic Phase?

Ultrafast phase transitions induced by femtosecond light pulses present a new opportunity for manipulating the properties of materials. Understanding how these transient states are different from, or similar to, their thermal counterparts is key to determining how materials can exhibit properties that are not found in equilibrium. In this paper, we reexamine the case of the light-induced insulator-metal phase transition in the prototypical, strongly correlated material VO$_2$, for which a nonthermal Mott-Hubbard transition has been claimed. Here, we show that heat, even on the ultrafast timescale, plays a key role in the phase transition. When heating is properly accounted for, we find a single phase-transition threshold corresponding to the thermodynamic structural insulator-metal phase transition, and we find no evidence of a hidden transient Mott-Hubbard nonthermal phase. The interplay between the initial thermal state and the ultrafast transition may have implications for other transient states of matter.

cond-mat.str-el↗

arXiv2020

Mapping nanoscale charge states and phase domains with quantitative hyperspectral coherent diffractive imaging spectroscopy

The critical properties of functional materials and nanoscale devices often originate from the coexistence of different thermodynamic phases and / or oxidization states, but sample makeup is seldom completely known a priori. Coherent diffractive imaging (CDI) provides the spatial resolution needed to observe nanoscale coexistence while returning the full amplitude and phase information of an object, but to date lacks the spectral information necessary for composition identification. Here we demonstrate CDI spectroscopy (CDIS), acquiring images of the prototypical quantum material vanadium oxide across the vanadium L2,3 and oxygen K X-ray absorption edges with nanometer scale resolution. Using the hyperspectral X-ray image we show coexistence of multiple oxidization states and phases in a single sample and extract the full complex refractive index of V2O5 and the monoclinic insulating and rutile conducting phases of VO2. These results constrain the role of hidden phases in the insulator-to-metal transition in VO2.

physics.optics↗

arXiv2020

Measurement of 10 fs pulses across the entire Visible to Near-Infrared Spectral Range

Tuneable ultrafast laser pulses are a powerful tool for measuring difficult-to-access degrees of freedom in materials science. In general these experiments require the ability to address resonances and excitations both above and below the bandgap of materials, and to probe their response at the timescale of the fastest non-trivial internal dynamics. This drives the need for ultrafast sources capable of delivering 10-15 fs duration pulses tuneable across the entire visible (VIS) and near infrared (NIR) range, 500 nm - 3000 nm, as well as the characterization of these sources. Here we present a single frequency-resolved optical gating (FROG) system capable of self-referenced characterization of pulses with 10 fs duration across the entire VIS-NIR spectral range. Our system does not require auxiliary beams and only minor reconfiguration for different wavelengths. We demonstrate the system with measurements of pulses across the entire tuning range.

physics.optics↗