arXiv subjects

Dragan Mihailovic

Publications and source records attributed to Dragan Mihailovic.

At least 19 recordsLinked to original sources

arXiv2026

Real-time imaging of quasiparticle dynamics at a topological defect in an electronic crystal

Defects formed during nonequilibrium self-assembly of quantum matter can dominate its emergent properties. Their macroscopic manifestations are typically characterised as noise, but their internal dynamics remain largely experimentally inaccessible. Making significant advances into the investigation of the microscopic degrees of freedom of such defects, we use fast scanning tunnelling microscopy to resolve, in real time, both internal and global dynamics of a mesoscopic Y-junction defect in an electronic crystal created through self-assembly after a local electromagnetic perturbation. We directly track individual electron rearrangements on millisecond timescales and map spatially localised telegraph noise characteristic of a two-level system. The phase and amplitude of these fluctuations are correlated with the observed charged particle trajectories, revealing a direct connection between collective order-parameter dynamics and microscopic charge motion. We model the dynamics as arising from the interplay of local Coulomb correlations, non-local configurational constraints and hybridised amplitude - phase collective modes bound to the junction. These constraints, together with the non-trivial broken symmetries of the defect, protect long-lived local quasiparticle configurations against external perturbations. Our results establish fast scanning tunnelling microscopy as a means of probing the internal dynamics of metastable quantum defects and reveal how microscopic correlations and collective modes are intertwined within topologically non-trivial structures in electronic crystals.

cond-mat.mes-hall↗

arXiv2026

A programmable superconductor created by light

The quest for superconductivity created by light extends for more than half a century, yet direct evidence of a true zero-resistance state - whose macroscopic quantum phase coherence is both created and controlled by light - has remained elusive. Here we report for the first time on a complex but robust light-programmable superconducting (LiPS) state at an aluminium-silicon heterojunction that is created and fully controlled with femtosecond laser pulses. The superconducting critical temperatures - ranging from 1.8 to 8.5 K, can be increased or erased at will by the application of tailored pulse sequences. At low temperatures the LiPS state shows features characteristic of a Berezinski-Kosterlitz-Thouless topological transition, but another distinct state appears at temperatures above 2 K, which shows clear signatures of quantum phase disorder. In the presence of a magnetic field we observe behaviour characteristic of vortex pinning and creep consistent with the 2-dimensional (2D) nature of the phase coherent system. The origin of the LiPS effect is attributed to light pulse control of the Moire-like superlattice of misfit dislocations (MDs) arising from discommensurations between the Al and Si lattices which is visible by high-resolution electron microscopy. We show how light pulses can be used to control the superlattice periodicity and highlight the appearance of topologically protected soliton-like kinks along the dislocation lines, important for imparting controllable metastability to the system. The demonstration of LiPS paves the way for designing metastable superconducting devices with controllable phase-coherence, enabling applications such as light-engineered quantum circuits, local gap tuning in quantum processors, and optically switchable superconducting devices.

cond-mat.supr-con↗

arXiv2025

Van der Waals devices for surface-sensitive experiments

In-operando characterization of van der Waals (vdW) devices using surface-sensitive methods provides critical insights into phase transitions and correlated electronic states. Yet, integrating vdW materials in functional devices while maintaining pristine surfaces is a key challenge for combined transport and surface-sensitive experiments. Conventional lithographic techniques introduce surface contamination, limiting the applicability of state-of-the-art spectroscopic probes. We present a stencil lithography-based approach for fabricating vdW devices, producing micron-scale electrical contacts, and exfoliation in ultra-high vacuum. The resist-free patterning method utilizes a shadow mask to define electrical contacts and yields thin flakes down to the single-layer regime via gold-assisted exfoliation. As a demonstration, we fabricate devices from 1$T-$TaS$_2$ flakes, achieving reliable contacts for application of electrical pulses and resistance measurements, as well as clean surfaces allowing for angle-resolved photoemission spectroscopy. The approach provides a platform for studying the electronic properties of vdW systems with surface-sensitive probes in well-defined device geometries.

cond-mat.mes-hall↗

arXiv2025

Nanocryotron-driven Charge Configuration Memristor

Cryo-computing - both classical and quantum, is severely limited by the absence of a suitable cryo-memory. The challenge both in terms of energy efficiency and speed have been known for decades, but so far conventional technologies have not been able to deliver adequate performance. Here we present a novel non-volatile memory device which incorporates a superconducting nanowire and an all-electronic charge configuration memristor (CCM) based on switching between charge-ordered states in a layered dichalcogenide material. We investigate the time-dynamics and current-voltage characteristics of such a device fabricated using a NbTiN nanowire and a 1T-TaS2 CCM. The observed dynamical response of the device is faithfully reproduced by modelling of the superconducting order parameter showing versatility of application. The inherent ultrahigh energy efficiency and speed of the device, which is compatible with single flux quantum logic, leads to a promising new memory concept for use in cryo-computing and quantum computing peripheral devices.

physics.comp-ph↗

arXiv2025

Imaging of electrically controlled van der Waals layer stacking in 1T-TaS2

Van der Waals (vdW) materials exhibit a variety of states that can be switched with low power at low temperatures, offering a viable cryogenic "flash memory" required for the classical control electronics for solid-state quantum information processing. In 1T-TaS2, a non-volatile metallic 'hidden' state can be induced from an insulating equilibrium charge-density wave ground state using either optical or electrical pulses. Given that conventional memristors form localized, filamentary channels which support the current, a key question for design concerns the geometry of the conduction region in highly energy-efficient 1T-TaS2 devices. Here, we report in operando micro-beam X-ray diffraction, fluorescence, and concurrent transport measurements, allowing us to spatially image the non-thermal hidden state induced by electrical switching of 1T-TaS2. Our results reveal a long-range ordered, non-filamentary switched state that extends well below the electrodes, implying that the self-organized, collective growth of the hidden phase is driven by a combination of charge flow and lattice strain. Our unique combination of techniques showcases the potential of non-destructive, three-dimensional X-ray imaging to study bulk switching properties in microscopic detail, namely electrical control of the vdW layer stacking.

cond-mat.str-el↗

arXiv2024

Non-equilibrium quantum domain reconfiguration dynamics in a two-dimensional electronic crystal: experiments and quantum simulations

Relaxation dynamics of complex many-body quantum systems brought out of equilibrium and subsequently trapped into metastable states is a very active field of research from both the theoretical and experimental point of view with implications in a wide array of topics from macroscopic quantum tunnelling and nucleosynthesis to non-equilibrium superconductivity and new energy-efficient memory devices. Understanding the dynamics of such systems is crucial for exploring fundamental aspects of many-body non-equilibrium quantum physics. In this work we investigate quantum domain reconfiguration dynamics in the electronic superlattice of a quantum material where classical dynamics is topologically constrained. The crossover from temperature to quantum fluctuation dominated dynamics in the context of environmental noise is investigated by directly observing charge reconfiguration with time-resolved scanning tunneling microscopy. The process is modelled using a programmable superconducting quantum simulator in which qubit interconnections correspond directly to the microscopic interactions between electrons in the quantum material. Crucially, the dynamics of both the experiment on the quantum material and the simulation is driven by spectrally similar pink noise. We find that the simulations reproduce the emergent time evolution and temperature dependence of the experimentally observed electronic domain dynamics remarkably well. The combined experiment and simulations lead to a better understanding of noise-driven quantum dynamics in open quantum systems. From a practical viewpoint, the results are important for understanding the origin of the retention time in non-volatile memory devices such as those based on 1T-TaS2.

quant-ph↗

arXiv2023

A high-efficiency programmable modulator for extreme ultraviolet light with nm feature size based on an electronic phase transition

The absence of efficient light modulators for extreme ultraviolet (EUV) and X-ray photons significantly limits their real-life application, particularly when even slight complexity of the beam patterns is required. Here we report on a novel approach to reversible imprinting of a holographic mask in an electronic Wigner crystal material with a sub-90 nm feature size. The structure is imprinted on a sub-picosecond time-scale using EUV laser pulses and acts as a high-efficiency diffraction grating that deflects EUV or soft X-ray light. The imprinted nanostructure is stable after the removal of the exciting beams at low temperatures but can be easily erased by a single heating beam. Modeling shows that the efficiency of the device can exceed 1%, approaching state-of-the-art etched gratings, but with the benefit of being programmable and tunable over a large range of wavelengths. The observed effect is based on the rapid change of lattice constant upon transition between metastable electronically-ordered phases in a layered transition metal dichalcogenide. The proposed approach is potentially useful for creating tunable light modulators in the EUV and soft X-ray spectral ranges.

physics.optics↗

arXiv2023

Cavity-mediated thermal control of metal-to-insulator transition in 1T-TaS$_{2}$

Placing quantum materials into optical cavities provides a unique platform for controlling quantum cooperative properties of matter, via both weak and strong light-matter coupling. Here we report the experimental evidence of reversible cavity control of a metal-to-insulator phase transition in a correlated solid-state material. We embed the charge density wave material 1T-TaS$_{2}$ into cryogenic tunable terahertz cavities and show that a switch between conductive and insulating behaviors, associated with a large change in the sample temperature, is obtained by mechanically tuning the distance between the cavity mirrors and their alignment. The large thermal modification observed is indicative of a Purcell-like scenario in which the spectral profile of the cavity modifies the energy exchange between the material and the external electromagnetic field. Our findings provide opportunities for controlling the thermodynamics and macroscopic transport properties of quantum materials by engineering their electromagnetic environment.

cond-mat.mtrl-sci↗

arXiv2023

Dynamics of non-thermal states in optimally-doped $Bi_2Sr_2Ca_{0.92}Y_{0.08}Cu_2O_{8+δ}$ revealed by mid-infrared three-pulse spectroscopy

In the cuprates, the opening of a d-wave superconducting (SC) gap is accompanied by a redistribution of spectral weight at energies two orders of magnitude larger than this gap. This indicates the importance to the pairing mechanism of on-site electronic excitations, such as orbital transitions or charge transfer excitations. Here, we resort to a three-pulse pump-probe scheme to study the broadband non-equilibrium dielectric function in optimally-doped $Bi_2Sr_2Ca_{0.92}Y_{0.08}Cu_2O_{8+δ}$ and we identify two distinct dynamical responses: i) a blueshift of the central energy of an interband excitation peaked at 2 eV and ii) a change in spectral weight in the same energy range. Photoexcitation with near-IR and mid-IR pulses, with photon energies respectively above and below the SC gap, reveals that the transient changes in the central energy are not modified by the onset of superconductivity and do not depend on the pump photon energy. Conversely, the spectral weight dynamics strongly depends on the pump photon energy and has a discontinuity at the critical temperature. The picture that emerges is that, while high-energy pulses excite quasiparticles in both nodal and thermally inaccessible antinodal states, photoexcitation by low-energy pulses mostly accelerates the condensate and creates excitations predominantly at the nodes of the SC gap. These results, rationalized by kinetic equations for d-wave superconducting gaps, indicate that dynamical control of the momentum-dependent distribution of non-thermal quasiparticles may be achieved by the selective tuning of the photoexcitation.

cond-mat.supr-con↗

arXiv2023

Unconventional photo-induced charge-density-wave dynamics in 2H-NbSe$_{2}$

We investigated temperature ($T$) dependent ultrafast near-infrared (NIR) transient reflectivity dynamics in coexisting superconducting (SC) and charge density wave (CDW) phases of two-dimensional 2H-NbSe$_{2}$ using NIR and visible excitations. With visible pump-photon excitation (400 nm) we find a slow high-energy quasiparticle relaxation channel which is present in all phases. In the CDW phase, we observe a distinctive transient response component, irrespective of the pump-photon energy. The component is marked by the absence of coherent amplitude mode oscillations and a relatively slow, picosecond rise time, which is different than in most of the typical CDW materials. In the SC phase, another tiny component emerges that is associated with optical suppression of the SC phase. The transient reflectivity relaxation in the CDW phase is dominated by phonon diffusive processes with an estimated low-$T$ heat diffusion constant anisotropy of $\sim30$. Strong excitation of the CDW phase reveals a weakly non-thermal CDW order parameter (OP) suppression. Unlike CDW systems with a larger gap, where the optical OP suppression involves only a small fraction of phonon degrees of freedom, the OP suppression in 2H-NbSe$_{2}$ is characterised by the excitation of a large amount of phonon degrees of freedom and significantly slower dynamics.

cond-mat.supr-con↗

arXiv2022

Ultra-Efficient Resistance Switching between Charge Ordered Phases in 1T-TaS$_2$ with a Single Picosecond Electrical Pulse

Progress in high-performance computing demands significant advances in memory technology. Among novel memory technologies that promise efficient device operation on a sub-ns timescale, resistance switching between charge ordered phases of the 1T-TaS$_2$ has shown to be potentially useful for the development of high-speed, energy efficient non-volatile memory device. While ultrafast switching was previously reported with optical pulses, determination of the intrinsic speed limits of actual devices that are triggered by electrical pulses is technically challenging and hitherto still largely unexplored. A new optoelectronic laboratory-on-a-chip, designed for measurements of ultrafast memory switching, enables an accurate measurement of the electrical switching parameters with 100 fs temporal resolution. A photoconductive response is used for ultrashort electrical pulse generation, while its propagation along a coplanar transmission line is detected using electro-optical sampling using a purpose-grown highly-resistive electro-optic (Cd,Mn)Te crystal substrate. By combining the transmission line and the 1T-TaS$_2$ device in a single optoelectronic circuit a non-volatile resistance switching with a single 1.9 ps electrical pulse is demonstrated, with an extremely small switching energy density per unit area E$_A$ = 9.4 fJ/$μ$m$^2$. The experiments demonstrate ultrafast, energy-efficient circuits utilizing switching between non-volatile charge-ordered states offers a new technological platform for cryogenic memory devices.

cond-mat.str-el↗

arXiv2022

Why does maximum Tc occur at the cross-over from weak to strong electron-phonon coupling in high temperature superconductors?

In cuprate superconductors, a pronounced maximum of superconducting Tc is observed in compounds that have an in-plane Cu-O distance close to ~1.92 Angstroms. On the other hand, direct measurements of the electron-phonon coupling lambda as a function of Cu-O distance show a clear linear correlation, implying that Tc is a strongly non-linear function of lambda. Conventional superconductivity theories based on the electron-phonon interaction predict a monotonic dependence of Tc on electron phonon coupling constant, which makes them incompatible with the observed behaviour. The observed cross-over behaviour as a function of lambda suggests that Tc occurs at the cross-over from weak to strong coupling, which is also associated with the onset of carrier localization. A coexistence, with a dynamical exchange of localized and itinerant carriers in a two-component superconductivity scenario are in agreement with the observed anomalous behavior and are suggested to be the key to understanding the mechanism for achieving high Tc.

cond-mat.supr-con↗

arXiv2021

Spatially intertwined superconductivity and charge order in 1T-TaSSe revealed by scanning tunneling spectroscopy

The interplay of different emergent phenomena -- superconductivity (SC) and domain formation -- appearing on different spatial and energy scales are investigated using high-resolution scanning tunneling spectroscopy (STS) in the prototypical transition metal dichalcogenide superconductor $1T$-TaSSe single crystals ($T_{SC} = 3.2$ K) at temperatures from 1 to 20 K. Our major observation is that while the SC gap size smoothly varies on the scale of $\lesssim 10$ nm, its spatial distribution is not correlated to the domain structure. On the other hand, there is statistically significant correlation of the SC gap $Δ_{SC}$ with spectral weight of the narrow band at the Fermi level formed from the same Ta $5d$ orbitals as the Mott-Hubbard band. We show that the narrow band follows the evolution of Hubbard bands in space, proving unambiguously its relation to the charge order. The correlations between the two suggest a non-trivial link between rapidly spatially varying charge order and superconductivity common in many quantum materials, and high-temperature superconductors in particular.

cond-mat.supr-con↗

arXiv2021

Energy efficient manipulation of topologically protected states in non-volatile ultrafast charge configuration memory devices

Non-volatile magnetic storage, from 1940s magnetic core to present day racetrack memory and magnetic anisotropy switching devices rely on the metastability of magnetic domains to store information. However, the inherent inefficiency of converting the information-carrying charge current into magnetization switching sets fundamental limitations in energy consumption. Other non-magnetic non-volatile memories such as memristors, ferroelectric memory and phase change memory devices also rely on energetically relatively costly crystal structural rearrangements to store information. In contrast, conventional electronic charge states in quantum dots for example, can be switched in femtoseconds with high efficiency, but any stored information dissipates rapidly. Here we present a radically different approach in the form of a charge-configuration memory (CCM) device that relies on charge-injection-driven electronic crystal melting and topological protection of the resulting electronic domain configurations of a two-dimensional electronic crystal to store information. With multiprobe scanning tunneling microscopy (STM) we show microscopically, within an operational device, how dislocations in the domain ordering lead to metastability by a mechanism that is topologically equivalent to magnetic bubble memory. The devices have a very small switching energy (<2.2 fJ/bit), ultrafast switching speed of <11 ps and operational range over more than 3 orders of magnitude in temperature (<250 mK ~ 190 K). Together with their simple functionality, a large resistance switching ratio, straightforward fabrication and impressive endurance, CCM devices introduce a new memory paradigm in emerging cryo-computing and other high-performance computing applications that require ultrahigh speed and low energy consumption.

cond-mat.str-el↗

arXiv2021

Quantum billiards with correlated electrons confined in triangular transition metal dichalcogenide monolayer nanostructures created by laser quench

Forcing systems though fast non-equilibrium phase transitions offers the opportunity to study new states of quantum matter that self-assemble in their wake. Here we study the quantum interference effects of correlated electrons confined in monolayer quantum nanostructures, created by femtosecond laser-induced quench through a first-order polytype structural transition in a layered transition-metal dichalcogenide material. Scanning tunnelling microscopy of the electrons confined within equilateral triangles, whose dimensions are a few crystal unit cells on the side, reveals that the trajectories are strongly modified from free-electron states both by electronic correlations and confinement. Comparison of experiments with theoretical predictions of strongly correlated electron behaviour reveals that the confining geometry destabilizes the Wigner/Mott crystal ground state, resulting in mixed itinerant and correlation-localized states intertwined on a length scale of 1 nm. Occasionally, itinerant-electron states appear to follow quantum interferences which are suggestive of classical trajectories (quantum scars). The work opens the path toward understanding the quantum transport of electrons confined in atomic-scale monolayer structures based on correlated-electron-materials.

cond-mat.str-el↗

arXiv2020

A time-domain phase diagram of metastable states in a charge ordered quantum material

Metastable self-organized electronic states in quantum materials are of fundamental importance, displaying emergent dynamical properties that may be used in new generations of sensors and memory devices. Such states are typically formed through phase transitions under non-equilibrium conditions and the final state is reached through processes that span a large range of timescales. By using time-resolved optical techniques and femtosecond-pulse-excited scanning tunneling microscopy (STM), the evolution of the metastable states in the quasi-two-dimensional dichalcogenide 1T-TaS2 is mapped out on a temporal phase diagram using the photon density and temperature as control parameters on timescales ranging from 10^(-12) to 10^3 s. The introduction of a time-domain axis in the phase diagram enables us to follow the evolution of metastable emergent states created by different phase transition mechanisms on different timescales, thus enabling comparison with theoretical predictions of the phase diagram and opening the way to understanding of the complex ordering processes in metastable materials.

cond-mat.str-el↗

arXiv2019

Unveiling the electronic transformations in the semi-metallic correlated-electron transitional oxide Mo$_8$O$_{23}$

Mo$_8$O$_{23}$ is a low-dimensional chemically robust transition metal oxide coming from a prospective family of functional materials, MoO$_{3-x}$, ranging from a wide gap insulator $(x=0)$ to a metal $(x=1)$. The large number of stoichometric compounds with intermediate $x$ have widely different properties. In Mo$_8$O$_{23}$, an unusual charge density wave transition has been suggested to occur above room temperature, but its low temperature behaviour is particularly enigmatic. We present a comprehensive experimental study of the electronic structure associated with various ordering phenomena in this compound, complemented by theory. Density-functional theory (DFT) calculations reveal a cross-over from a semi-metal with vanishing band overlap to narrow-gap semiconductor behaviour with decreasing temperature. A buried Dirac crossing at the zone boundary is confirmed by angle-resolved photoemission spectroscopy (ARPES). Tunnelling spectroscopy (STS) reveals a gradual gap opening corresponding to a metal-to-insulator transition at 343 K in resistivity, consistent with CDW formation and DFT results, but with large non-thermal smearing of the spectra implying strong carrier scattering. At low temperatures, the CDW picture is negated by the observation of a metallic Hall contribution, a non-trivial gap structure in STS below $\sim 170$ K and ARPES spectra, that together represent evidence for the onset of the correlated state at $70$ K and the rapid increase of gap size below $\sim 30$ K. The intricate interplay between electronic correlations and the presence of multiple narrow bands near the Fermi level set the stage for metastability and suggest suitability for memristor applications.

cond-mat.str-el↗

arXiv2019

Time-resolved reflectivity and Raman studies of the interplay of enigmatic orders in Mo$_8$O$_{23}$

Monoclinic semi-metallic Mo$_8$O$_{23}$ belongs to a multifunctional series of compounds showing multiple ordering phenomena that have not achieved much attention till now. Previous X-rays studies of this compound have revealed an incommensurate ordering transition at $T_{\mathrm{IC}}\sim350$ K, followed by a structural transition to commensurate order at $T_{\mathrm{IC-C}}=285$ K. In addition, an enigmatic resistance maximum is observed at $T_{\mathrm{el}}\sim150$ K, whose origin has so far proved elusive. Aiming to disentangle these multiple orders we use the polarized transient optical spectroscopy supplemented by Raman spectroscopy to study the electronic relaxation dynamics and lattice vibrational modes in Mo$_8$O$_{23}$ single crystals. Remarkably, both the coherent vibrational mode response and single particle response display extrema of damping/relaxation times close to $T_{\mathrm{el}}$ with the concurrent appearance of new coherent vibrational modes and a characteristic polarization asymmetry which saturates below $T_{\mathrm{el}}$. The single-particle relaxation data analysis shows the appearance of a temperature-independent gap in the electronic excitation spectrum below $T_{\mathrm{IC}}$ and additional temperature-dependent gap opening near $T_{\mathrm{el}}$. Concurrently, a low frequency vibrational mode shows anomalous softening around $T_{\mathrm{m}}\sim200$ K, far below $T_{\mathrm{IC-C}}$ and $T_{\mathrm{IC}}$. The observations are interpreted in terms of the appearance of a hidden gapped state below $T_{\mathrm{el}}$ that has so far eluded detection by structural analyses.

cond-mat.str-el↗