arXiv subjects

Dino Novko

Publications and source records attributed to Dino Novko.

At least 19 recordsLinked to original sources

arXiv2026

Phonon-induced pseudogap phase in TiSe$_2$

To comprehend quantum ordered states, such as charge density waves (CDW), in layered transition metal dichalcogenides (TMDCs), it is essential to uncover their underlying normal states. Here, we use time- and angle-resolved extreme ultraviolet photoemission spectroscopy and ab initio electron-phonon calculations to perform excited state band mapping of three prototypical 1T TMDCs, i.e., TiSe$_2$, HfTe$_2$, and ZrTe$_2$, at room temperature. The results reveal the profound impact of strong electron-phonon-induced thermal fluctuations on the normal-phase electronic structure. Specifically, in the moderate electron-phonon coupling regime, as in HfTe$_2$ and ZrTe$_2$, thermal fluctuations only lead to small spectral broadening and band renormalization. In the strongly coupled case, exemplified by TiSe$_2$, we observe soft-phonon-induced, momentum-dependent suppression of spectral weight, i.e., pseudogaps - extending up to 1 eV above the Fermi level. Our work establishes the normal phase of TiSe$_2$ as a phonon-induced pseudogap phase governed by strong CDW fluctuations, thereby uncovering previously missing aspects of the TiSe$_2$ phase diagram, with broader implications for other TMDCs in the strong electron-phonon coupling regime.

cond-mat.str-el↗

arXiv2026

Fluctuation-driven multi-step charge density wave transition in monolayer TiSe$_2$

The exact microscopic origin, symmetry, and thermal melting mechanism of the charge density wave (CDW) phase in TiSe$_{2}$ remain a subject of intense debate, particularly regarding the presence of chiral structural order and a multi-step phase transition. Here, we resolve the finite-temperature structural dynamics of the monolayer TiSe$_{2}$ using large-scale molecular dynamics simulations driven by an accurate, first-principles-trained machine-learning interatomic potential. We demonstrate that the CDW melting deviates from a conventional second-order phase transition, while it undergoes a two-step melting process characterised by an extended fluctuation regime between $T^{\ast}\approx200$ K and $T_{\mathrm{CDW}}\approx250$ K, with proliferation of topological defects and domain walls, and accompanied by a completely overdamped soft optical phonon. Furthermore, we reveal that anisotropic long-wavelength thermal fluctuations spontaneously stabilise an asymmetric $3Q$ chiral CDW order with $C2$ symmetry. These findings provide a unified microscopic framework for understanding complex fluctuation-driven phase transitions in 2D quantum materials, demonstrating that the intricate CDW physics of TiSe$_{2}$ can be largely captured without invoking excitonic correlations.

cond-mat.mtrl-sci↗

arXiv2026

Unconventional plasmon dynamics due to strong correlations in Sr$_2$RuO$_4$

Plasmon modes, their dispersion, and the onset of damping when approaching the electron-hole continuum are well understood when electron correlations are weak. However, we know little about how this picture is modified and what additional features emerge in strongly correlated materials. Here, we present a fully ab initio approach to plasmon excitations that combines density functional theory with dynamical mean-field theory, and we use it to reconcile controversial electron energy-loss spectroscopy results in Sr$_2$RuO$_4$. In particular, we show that electronic correlations reproduce the plasmon dispersion, while generating a large intrinsic width already below the electron-hole continuum. An additional high-energy peak reflecting transitions between incoherent features and a sharp increase of the plasmon's energy-momentum dispersion, akin to waterfalls in photoemission spectroscopy, are identified as genuine correlation effects.

cond-mat.str-el↗

arXiv2025

A Symmetric Superconducting Dome Hosts Non-Fermi Liquid Behavior at Optimal Doping in MoS2

The similarities between the phase diagrams of ionic liquid-gated transition metal dichalcogenides (TMDCs) and high-temperature superconductors have garnered considerable interest due to the presence of a superconducting dome with a non-monotonic dependence of the superconducting order parameter as a function of charge carrier density. However, the lack of a complete superconducting dome and insights into the normal state in ionic liquid-gated TMDCs prevents a detailed mapping between the two systems. In this work, we obtain a symmetric superconducting dome that extends from deep underdoped regime all the way to deep overdoped regime in ionic liquid gated MoS2 with a refined gating protocol. We demonstrate that the dome is anticorrelated with the evolution of non-Fermi liquid behavior in the normal states. The scattering rate in the non-Fermi liquid regime can reach Planckian limit. The results may shed light on the emergence of superconductivity in TMDCs.

cond-mat.supr-con↗

arXiv2025

Excitons and Optical Response in Excitonic Insulator Candidate TiSe$_2$

The origin of the charge density wave (CDW) phase in TiSe$_2$ is a highly debated topic, with lattice and excitonic correlations proposed as the main driving mechanisms. One of the proposed scenarios is the excitonic insulator (EI) mechanism, where soft electronic mode drives the phase transition. However, the existence of this purely electronic mode is controversial. Here, we perform fully ab-initio calculations of the electron excitation spectra in TiSe$_2$ with electron-hole excitonic effects included via Bethe-Salpeter equations. In the normal high-temperature phase the excitation spectra is dominated by the exciton mode at 1.6 eV, while no well-defined soft electronic modes that could support the EI phase are present. In the CDW phase, the structural distortions induce a CDW band-gap opening between Ti-$d$ and Se-$p$ states, which supports the formation of the two low-energy excitonic modes in the optical spectrum at 0.4 eV and 80 meV. Close to the transition temperature $T_{\rm CDW}$, these two excitonic modes are softened and approach zero energy. These results suggest that the EI mechanism is not a main driving force in the formation of the CDW phase in TiSe$_2$, but there is a region in the phase diagram near $T_{\rm CDW}$ where EI fluctuations could be relevant.

cond-mat.str-el↗

arXiv2025

Ultrafast Nonequilibrium Enhancement of Electron-Phonon Interaction in 2H-MoTe$_2$

Understanding nonequilibrium electron-phonon interactions at the microscopic level and on ultrafast timescales is a central goal of modern condensed matter physics. Combining time- and angle-resolved extreme ultraviolet photoemission spectroscopy with constrained density functional perturbation theory, we demonstrate that photoexcited carrier density can serve as a tuning knob to enhance electron-phonon interactions in nonequilibrium conditions. Specifically, nonequilibrium band structure mapping and valley-resolved ultrafast population dynamics in semiconducting transition-metal dichalcogenide 2H-MoTe$_2$ reveal band-gap renormalizations and reduced population lifetimes as photoexcited carrier densities increase. Through theoretical analysis of photoinduced electron and phonon energy and linewidth renormalizations, we attribute these transient features to nonequilibrium modifications of electron-phonon coupling matrix elements. The present study advances our understanding of microscopic coupling mechanisms enabling control over relaxation pathways in driven solids.

cond-mat.mtrl-sci↗

arXiv2025

Electron-phonon-dominated charge-density-wave fluctuations in TiSe$_2$ accessed by ultrafast nonequilibrium dynamics

The complex phase diagram of 1T-TiSe2 consists of a charge density wave (CDW) below 200 K, and CDW fluctuations of still unknown origin at higher temperatures. Here, we use time-resolved extreme ultraviolet momentum microscopy and density functional perturbation theory to uncover the formation mechanism of CDW fluctuations and their spectral features at 295 K. We investigated the transient dynamics of fluctuations upon nonresonant ultrafast photoexcitation, and directly correlate it with the CDW soft-phonon hardening. Surprisingly, our results show that the coherent amplitude mode modulating ultrafast CDW recovery persists above TCDW, and reveal that CDW fluctuations are dominated by the electron-phonon interaction rather than excitonic correlations as commonly believed. Our findings on these microscopic CDW fluctuations clarify the complex interplay between electronic and lattice degrees of freedom at elevated temperatures and, therefore, could be useful in understanding the nature of the CDW phase transition in 1T-TiSe2 and similar quantum materials.

cond-mat.str-el↗

arXiv2025

Unconventional Materials for Light Dark Matter Detection

We propose the use of several unconventional materials as detectors for dark matter with mass beneath the MeV scale. These include the transition-metal dichalcogenide TiSe$_2$ hosting a low-energy plasmon in the charge-density-wave phase, Sr$_2$RuO$_4$ containing a low-energy acoustic demon mode, and hole-doped diamond with tunable optical and acoustic plasmon frequencies. We perform first-principles density functional theory computations of their loss functions at non-vanishing momenta and establish their reach into light dark matter parameter space. We show that due to intense low-energy plasmon modes -- of different microscopic origin in each -- the reach of detectors based on these materials could surpass existing proposals by several orders of magnitude for both dark matter scattering and absorption on electrons. The anisotropic response of these materials, which enables directional detection, renders them exceptionally strong detector candidates, motivating the design and fabrication of future devices.

hep-ph↗

arXiv2025

Machine learning model for efficient nonthermal tuning of the charge density wave in monolayer NbSe$_2$

Understanding and controlling the charge density wave (CDW) phase diagram of transition metal dichalcogenides is a long-studied problem in condensed matter physics. However, due to complex involvement of electron and lattice degrees of freedom and pronounced anharmonicity, theoretical simulations of the CDW phase diagram at the density-functional-theory level are often numerically demanding. To reduce the computational cost of first principles modelling by orders of magnitude, we have developed an electronic free energy machine learning model for monolayer NbSe$_2$ that allows changing both electronic and ionic temperatures independently. Our approach relies on a machine learning model of the electronic density of states and zero-temperature interatomic potential. This allows us to explore the CDW phase diagram of monolayer NbSe$_2$ both under thermal and laser-induced nonthermal conditions. Our study provides an accurate estimate of the CDW transition temperature at low cost and can disentangle the role of hot electrons and phonons in nonthermal ultrafast melting process of the CDW phase in NbSe$_2$.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast dynamics of vibronically dressed core-excitons in graphite: a femtosecond RIXS perspective

This study demonstrates one of the first implementations of time-resolved resonant inelastic X-ray scattering (tr-RIXS), marking a seminal extension of RIXS spectroscopy into the ultrafast time domain. By investigating the ultrafast dynamics of vibronically dressed core excitons in graphite using femtosecond X-ray pulses from a Free Electron Laser, we reveal previously inaccessible insights into the transient coupling between core excitons and specific optical phonon modes. Our approach establishes tr-RIXS as a powerful, transformative tool capable of elucidating the intricate interplay between electronic and lattice dynamics, opening new avenues in ultrafast materials research.

cond-mat.mtrl-sci↗

arXiv2024

Understanding the origin of superconducting dome in electron-doped MoS$_2$ monolayer

We investigate the superconducting properties of molybdenum disulphide (MoS$_2$) monolayer across a broad doping range, successfully recreating the so far unresolved superconducting dome. Our first-principles findings reveal several dynamically stable phases across the doping-dependent phase diagram. We observe a doping-induced increase in the superconducting transition temperature $T_c$, followed by a reduction in $T_c$ due to the formation of charge density waves (CDWs), polaronic distortions, and structural transition from the H to the 1T$'$ phase. Our work reconciles various experimental observations of CDWs in MoS$_2$ with its doping-dependent superconducting dome structure, which occurs due to the $1\times 1$ H to $2\times 2$ CDW phase transition.

cond-mat.supr-con↗

arXiv2024

Electron-mediated anharmonicity and its role in the Raman spectrum of graphene

The Raman active G mode in graphene exhibits strong coupling to electrons, yet the comprehensive treatment of this interaction in the calculation of its temperature-dependent Raman spectrum remains incomplete. In this study, we calculate the temperature dependence of the G mode frequency and linewidth, and successfully explain the experimental trend, by accounting for the contributions arising from the first-order electron-phonon coupling, electron-mediated phonon-phonon coupling, and standard lattice anharmonicity. The generality of our approach enables its broad applicability to study phonon dynamics in materials where both electron-phonon coupling and anharmonicity are important.

cond-mat.mtrl-sci↗

arXiv2024

Unconventional Thermophotonic Charge Density Wave

Charge-order states of broken symmetry, such as charge density wave (CDW), are able to induce exceptional physical properties, however, the precise understanding of the underlying physics is still elusive. Here, we combine fluctuational electrodynamics and density functional theory to reveal an unconventional thermophotonic effect in CDW-bearing TiSe$_2$, referred to as thermophotonic-CDW ($tp$-CDW). The interplay of plasmon polariton and CDW electron excitations give rise to an anomalous negative temperature dependency in thermal photons transport, offering an intuitive fingerprint for a transformation of the electron order. Additionally, the demonstrated nontrivial features of $tp$-CDW transition hold promise for a controllable manipulation of heat flow, which could be extensively utilized in various fields such as thermal science and electron dynamics, as well as in next-generation energy devices.

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity in 2D systems enhanced by nonadiabatic phonon-production effects

We investigate the dynamical effects of electron-phonon coupling (EPC) on the superconducting properties of two-dimensional (2D) systems, calculating the Eliashberg function in terms of dynamically renormalized phonons. By studying different approximations for the phonon self-energy, we identify the important role of charge fluctuations in shaping the superconductivity properties, not only through the renormalization of phonon frequencies and damping rates but also through structural changes in the phonon spectral function. With the dynamical effects treated consistently, we argue that a part of the phonon spectral weight necessarily shifts to low frequencies due to the coupling to the 2D gapless plasmon. Furthermore, we find that the EPC leads to excess phonon spectral weight as well - i.e., phonon production - which generally tends to enhance the transition temperature. Our calculations point out that the influence of phonon production becomes greater as the density and the effective mass of electrons increase.

cond-mat.supr-con↗

arXiv2023

Dynamical Phonons Following Electron Relaxation Stages in Photo-excited Graphene

Ultrafast electron-phonon relaxation dynamics in graphene hides many distinct phenomena, such as hot phonon generation, dynamical Kohn anomalies, and phonon decoupling, yet still remains largely unexplored. Here, we unravel intricate mechanisms governing the vibrational relaxation and phonon dressing in graphene at a highly non-equilibrium state by means of first-principles techniques. We calculate dynamical phonon spectral functions and momentum-resolved linewidths for various stages of electron relaxation and find photo-induced phonon hardening, overall increase of relaxation rate and nonadiabaticity as well as phonon gain. Namely, the initial stage of photo-excitation is found to be governed by strong phonon anomalies of finite-momentum optical modes along with incoherent phonon production. Population inversion state, on the other hand, allows production of coherent and strongly-coupled phonon modes. Our research provides vital insights into the electron-phonon coupling phenomena in graphene, and serves as a foundation for exploring non-equilibrium phonon dressing in materials where ordered states and phase transitions can be induced by photo-excitation.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast nonadiabatic phonon renormalization in photoexcited single-layer MoS$_2$

Comprehending nonequilibrium electron-phonon dynamics at the microscopic level and at the short time scales is one of the main goals in condensed matter physics. Effective temperature models and time-dependent Boltzmann equations are standard techniques for exploring and understanding nonequilibrium state and the corresponding scattering channels. However, these methods consider only the time evolution of carrier occupation function, while the self-consistent phonon dressing in each time instant coming from the nonequilibrium population is ignored, which makes them less suitable for studying ultrafast phenomena where softening of the phonon modes plays an active role. Here, we combine ab-initio time-dependent Boltzmann equations and many-body phonon self-energy calculations to investigate the full momentum- and mode-resolved nonadiabatic phonon renormalization picture in the MoS$_2$ monolayer under nonequilibrium conditions. Our results show that the nonequilibrium state of photoexcited MoS$_2$ is governed by multi-valley topology of valence and conduction bands that brings about characteristic anisotropic electron-phonon thermalization paths and the corresponding phonon renormalization of strongly-coupled modes around high-symmetry points of the Brillouin zone. As the carrier population is thermalized towards its equilibrium state, we track in time the evolution of the remarkable phonon anomalies induced by nonequilibrium and the overall enhancement of the phonon relaxation rates. This work shows potential guidelines to tailor the electron-phonon relaxation channels and control the phonon dynamics under extreme photoexcited conditions.

cond-mat.mtrl-sci↗

arXiv2023

Plasmon Excitations Across the Charge-Density-Wave Transition in Single-Layer TiSe$_2$

$1T$-TiSe$_2$ is believed to posses a soft electronic mode, i.e., plasmon or exciton, that might be responsible for the exciton condensation and charge-density-wave (CDW) transition. Here, we explore collective electronic excitations in single-layer $1T$-TiSe$_2$ by using the ab-initio electromagnetic linear response and unveil intricate scattering pathways of two-dimensional (2D) plasmon mode near the CDW phase. We found the dominant role of plasmon-phonon scattering, which in combination with the CDW gap excitations leads to the anomalous temperature dependence of the plasmon linewidth across the CDW transition. Below the transition temperature $T_{\rm CDW}$ a strong hybridization between 2D plasmon and CDW excitations is obtained. These optical features are highly tunable due to temperature-dependent CDW-related modifications of electronic structure and electron-phonon coupling and make CDW-bearing systems potentially interesting for applications in optoelectronics and low-loss plasmonics.

cond-mat.mtrl-sci↗

arXiv2023

Vibrational dynamics of CO on Pd(111) in and out of thermal equilibrium

Using many-body perturbation theory and density functional perturbation theory, we study the vibrational spectra of the internal stretch (IS) mode of CO on Pd(111) for the bridge and hollow adsorption structures that are experimentally identified at 0.5~ML coverage. Our theoretical treatment allows us to determine the temperature dependence of the IS vibrational spectra under thermal conditions as well as the time evolution of the non-equilibrium transient spectra induced by femtosecond laser pulses. Under thermal conditions (i.e., for equal electronic $T_e$ and phononic $T_l$ temperatures), the calculated lifetimes at 10-150~K are mostly due to nonadiabatic couplings (NC), i.e., first-order electronic excitations. As temperature increases, also the contribution of the second-order electron mediated phonon-phonon couplings (EMPPC) progressively increases from 25\% at low temperatures to 50\% at 300~K. Our calculations for the laser-induced non-equilibrium conditions comprise experimental absorbed fluences of 6-130~J/m$^2$. For fluences for which $T_e>$2000~K, the transient vibrational spectra are characterized by two different regimes that follow the distinct time-evolution of $T_e$ and $T_l$ and are respectively dominated by NC and EMPPC processes. At lower fluences, the initial fast regime becomes progressively negligible as $T_e$ decreases and only the steady second regime remains visible. Qualitatively, all these spectral properties are common to the both adsorption structures studied here.

cond-mat.mtrl-sci↗