arXiv subjects

V. Esposito

Publications and source records attributed to V. Esposito.

At least 19 recordsLinked to original sources

arXiv2026

Persistent short-range charge correlations revealed by ultrafast melting of electronic order in YBa$_2$Cu$_3$O$_{6+x}$

Charge density waves (CDW) are ubiquitous in the complex phase diagram of cuprate superconductors and exhibit both short- and long-range correlations. Using time-resolved resonant X-ray scattering, we investigate the photo-induced dynamics of CDW in YBa$_2$Cu$_3$O$_{6.67}$. We discover an excitation threshold ($\Phi$$_\mathrm{C}$ $\approx$ 65 $\mu$J/cm$^2$) above which long-range CDW disappear, revealing a persistent CDW peak with short-range correlation length. Ultrafast photo-excitation promptly uncovers this residual short-range CDW correlations, appearing within $\approx$ 0.2 ps. Long-range CDW coherence recovers within $\approx$ 0.6 ps, while the peak intensity remains partially suppressed. We rationalize the dichotomic behavior in the fluence and temporal dependencies as the signature of two coexisting CDW peaks, arising from short- and long-range correlations, which we disentangle through their distinct response to photo-excitation. We provide evidence that the collapse of long-range correlations is driven by an electronic process, while short-range correlations are characterized by distinct timescales and stiffness against photo-excitation. This approach establishes ultrafast X-ray scattering as an effective tool for disentangling coexisting density waves and correlations in quantum materials.

cond-mat.supr-con↗

arXiv2024

Imaging ultrafast electronic domain fluctuations with X-ray speckle visibility

Speckle patterns manifesting from the interaction of coherent X-rays with matter offer a glimpse into the dynamics of nanoscale domains that underpin many emergent phenomena in quantum materials. While the dynamics of the average structure can be followed with time-resolved X-ray diffraction, the ultrafast evolution of local structures in nonequilibrium conditions have thus far eluded detection due to experimental limitations, such as insufficient X-ray coherent flux. Here we demonstrate a nonequilibrium speckle visibility experiment using a split-and-delay setup at an X-ray free-electron laser. Photoinduced electronic domain fluctuations of the magnetic model material Fe$_{3}$O$_{4}$ reveal changes of the trimeron network configuration due to charge dynamics that exhibit liquid-like fluctuations, analogous to a supercooled liquid phase. This suggests that ultrafast dynamics of electronic heterogeneities under optical stimuli are fundamentally different from thermally-driven ones.

cond-mat.str-el↗

arXiv2024

Nematicity of a Magnetic Helix

A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficulty of spin nematics is the unambiguous realization and characterization of the phase. Here we present an entirely new type of magnetic nematic phase, which replaces the basis of individual spins with magnetic helices. The helical basis allows for the direct measurement of the order parameters with soft X-ray scattering and a thorough characterization of the nematic phase and its thermodynamic transitions. We discover two distinct nematic phases with unique spatio-temporal correlation signatures. Using coherent X-ray methods, we find that near the phase boundary between the two nematic phases, fluctuations coexist on the timescale of both seconds and sub-nanoseconds. Additionally, we have determined that the fluctuations occur simultaneously with a reorientation of the magnetic helices, indicating that there is spontaneous symmetry breaking and new degrees of freedom become available. Our results provide a novel framework for characterizing exotic phases and the phenomena presented can be mapped onto a broad class of physical systems.

cond-mat.mtrl-sci↗

arXiv2023

Interplay between atomic fluctuations and charge density waves in La$_{2-x}$Sr$_{x}$CuO$_{4}$

In the cuprate superconductors, the spatial coherence of the charge density wave (CDW) state grows rapidly below a characteristic temperature $T_\mathrm{CDW}$, the nature of which is debated. We have combined a set of x-ray scattering techniques to study La$_{1.88}$Sr$_{0.12}$CuO$_{4}$ ($T_\mathrm{CDW}$~$\approx$~80\,K) to shed light on this discussion. We observe the emergence of a crystal structure, which is consistent with the CDW modulation in symmetry, well above $T_\mathrm{CDW}$. This global structural change also induces strong fluctuations of local atomic disorder in the intermediate temperature region. At $T_\mathrm{CDW}$, the temperature dependence of this structure develops a kink, while the atomic disorder is minimized. We find that the atomic relaxation dynamics cross over from a cooperative to an incoherent response at $T_\mathrm{CDW}$. These results reveal a rich interplay between the CDWs and atomic fluctuations of distinct spatio-temporal scales. For example, the CDW coherence is enhanced on quasi-elastic timescales by incoherent atomic relaxation.

cond-mat.supr-con↗

arXiv2022

Induced Giant Piezoelectricity in Centrosymmetric Oxides

Piezoelectrics are materials that linearly deform in response to an applied electric field. As a fundamental prerequisite, piezoelectric material must possess a non centrosymmetric crystal structure. For more than a century, this remains the major obstacle for finding new piezoelectric materials. We circumvent this limitation by breaking the crystallographic symmetry, and inducing large and sustainable piezoelectric effects in centrosymmetric materials by electric field induced rearrangement of oxygen vacancies Surprisingly, the results show the generation of extraordinarily large piezoelectric responses d33 ~200,000 pm/V), in cubic fluorite Gd-doped CeO2-x films, which is two orders of magnitude larger than in the presently best known lead based piezoelectric relaxor ferroelectric oxide. These findings open opportunities to design new piezoelectric materials from environmentally friendly centrosymmetric ones.

cond-mat.mtrl-sci↗

arXiv2021

Nonequilibrium Charge-Density-Wave Order Beyond the Thermal Limit

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system's coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

cond-mat.mtrl-sci↗

arXiv2020

Strain Wave Pathway to Semiconductor-to-Metal Transition revealed by time resolved X-ray powder diffraction

Thanks to the remarkable developments of ultrafast science, one of today's challenges is to modify material state by controlling with a light pulse the coherent motions that connect two different phases. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a macroscopic transformation pathway for the semiconducting-to-metal transition with large volume change in bistable Ti$_3$O$_5$ nanocrystals. Femtosecond powder X-ray diffraction allowed us to quantify the structural deformations associated with the photoinduced phase transition on relevant time scales. We monitored the early intra-cell distortions around absorbing metal dimers, but also long range crystalline deformations dynamically governed by acoustic waves launched at the laser-exposed Ti$_3$O$_5$ surface. We rationalize these observations with a simplified elastic model, demonstrating that a macroscopic transformation occurs concomitantly with the propagating acoustic wavefront on the picosecond timescale, several decades earlier than the subsequent thermal processes governed by heat diffusion.

cond-mat.mtrl-sci↗

arXiv2019

Aqueous metal-organic solutions for YSZ thin film inkjet deposition

Inkjet printing of 8% Y2O3-stabilized ZrO2 (YSZ) thin films is achieved by designing a novel water-based reactive ink for Drop-on-Demand (DoD) inkjet printing. The ink formulation is based on a novel chemical strategy that consists of a combination of metal oxide precursors (zirconium alkoxide and yttrium salt), water and a nucleophilic agent, i.e. n-methyldiethanolamine (MDEA). This chemistry leads to metal-organic complexes with long term ink stability and high precision printability. Ink rheology and chemical reactivity are analyzed and controlled in terms of metal-organic interactions in the solutions. Thin dense nanocrystalline YSZ film below 150 nm are obtained by low temperature calcination treatments (400-500 °C), making the deposition suitable for a large variety of substrates, including silicon, glass and metals. Thin films and printed patterns achieve full densification with no lateral shrinkage and high ionic conductivity.

physics.app-ph↗

arXiv2019

Zirconia nano-colloids transfer from continuous hydrothermal synthesis to inkjet printing

Water dispersion of nanometric yttria stabilized zirconia (YSZ) particles synthesized by continuous hydrothermal synthesis are transferred into nanoinks for thin film deposition. YSZ nanoparticles are synthesized in supercritical conditions resulting in highly dispersed crystals of 10 nm in size. The rheology of the colloid is tailored to achieve inkjet printability (Z) by using additives for regulating viscosity and surface tension. Inks with a wide range of properties are produced. A remarkable effect of nanoparticles on the ink printability is registered even at solid load < 1% vol. In particular, nanoparticles hinder the droplet formation at low values of the printability while suitable jetting is observed at high Z values, i.e. Z ca. 20. For the optimized inks, we achieve high quality printing with lateral and thickness resolution of 70 um and ca. 250 nm respectively, as well as self levelling effect with a reduction of the substrate roughness. Densification is achieved at sintering temperatures below 1200 C.

physics.app-ph↗

arXiv2019

Zirconia UV-curable colloids for additive manufacturing via hybrid inkjet printing-stereolithography

Currently, additive manufacturing of ceramics by stereolithography (SLA) is limited to single materials and by a poor thickness resolution that strongly depends on the ceramic particles-UV light interaction. Combining selective laser curing with inkjet printing represents a novel strategy to overcome these constrains. Nonetheless, this approach requires UV-curable inks that allow hardening of the printed material and sintering to high density. In this work, we report how to design an ink for inkjet printing of yttria stabilized zirconia (YSZ) which can be impressed by addition of UV-curable monomers. We especially show how the formulation of the inks and particularly the UV-monomer concentration impacts the printability and the UV-curing. This leads to prints that are resistant to solvent washing first and densify to 96% dense YSZ layers after sintering.

physics.app-ph↗

arXiv2019

Optical control of vibrational coherence triggered by an ultrafast phase transition

Femtosecond time-resolved x-ray diffraction is employed to study the dynamics of the periodic lattice distortion (PLD) associated with the charge-density-wave (CDW) in K0.3MoO3. Using a multi-pulse scheme we show the ability to extend the lifetime of coherent oscillations of the PLD about the undistorted structure through re-excitation of the electronic states. This suggests that it is possible to enter a regime where the symmetry of the potential energy landscape corresponds to the high symmetry phase but the scattering pathways that lead to the damping of coherent dynamics are still controllable by altering the electronic state population. The demonstrated control over the coherence time offers new routes for manipulation of coherent lattice states.

cond-mat.str-el↗

arXiv2019

Ultrafast transient increase of oxygen octahedral rotations in a perovskite

The ability to control the structure of a crystalline solid on ultrafast timescales bears enormous potential for information storage and manipulation or generating new functional states of matter [1]. In many materials where the ultrafast control of crystalline structures has been explored, optical excitation pushes materials towards their less ordered high temperature phase [2{9] as electronically driven ordered phases melt and possible concomitant structural modifications relax. Nonetheless, for a few select materials it has been shown that photoexcitation can slightly enhance the amplitude of an electronic ordering phenomenon (i.e. its electronic order parameter) [9{13]. Here we show via femtosecond hard X-ray diffraction that photodoping of the perovskite EuTiO3 transiently increases the order parameter associated with a purely structural [14] phase transition represented by the antiferrodistortive rotation of the oxygen octahedra. This can be understood from an ultrafast charge-transfer induced reduction of the Goldschmidt tolerance factor [15], which is a fundamental control parameter for the properties of perovskites

cond-mat.str-el↗

arXiv2018

Ultrafast relaxation dynamics of the antiferrodistortive phase in Ca doped SrTiO3

The ultrafast dynamics of the octahedral rotation in Ca:SrTiO3 is studied by time resolved x-ray diffraction after photo excitation over the band gap. By monitoring the diffraction intensity of a superlattice reflection that is directly related to the structural order parameter of the soft-mode driven antiferrodistortive phase in Ca:SrTiO3, we observe a ultrafast relaxation on a 0.2 ps timescale of the rotation of the oxygen octahedron, which is found to be independent of the initial temperaure despite large changes in the corresponding soft-mode frequency. A further, much smaller reduction on a slower picosecond timescale is attributed to thermal effects. Time-dependent density-functional-theory calculations show that the fast response can be ascribed to an ultrafast displacive modification of the soft-mode potential towards the normal state, induced by holes created in the oxygen 2p states.

cond-mat.mtrl-sci↗

arXiv2017

The photoinduced transition in magnetoresistive manganites: a comprehensive view

We use femtosecond x-ray diffraction to study the structural response of charge and orbitally ordered Pr$_{1-x}$Ca$_x$MnO$_3$ thin films across a phase transition induced by 800 nm laser pulses. By investigating the dynamics of both superlattice reflections and regular Bragg peaks, we disentangle the different structural contributions and analyze their relevant time-scales. The dynamics of the structural and charge order response are qualitatively different when excited above and below a critical fluence $f_c$. For excitations below $f_c$ the charge order and the superlattice is only partially suppressed and the ground state recovers within a few tens of nanosecond via diffusive cooling. When exciting above the critical fluence the superlattice vanishes within approximately half a picosecond followed by a change of the unit cell parameters on a 10 picoseconds time-scale. At this point all memory from the symmetry breaking is lost and the recovery time increases by many order of magnitudes due to the first order character of the structural phase transition.

cond-mat.str-el↗

arXiv2017

Domain size effects on the dynamics of a charge density wave in 1T-TaS2

Recent experiments have shown that the high temperature incommensurate (I) charge density wave (CDW) phase of 1T-TaS2 can be photoinduced from the lower temperature, nearly commensurate (NC) CDW state. Here we report a time-resolved x-ray diffraction study of the growth process of the photoinduced I-CDW domains. The layered nature of the material results in a marked anisotropy in the size of the photoinduced domains of the I-phase. These are found to grow self-similarly, their shape remaining unchanged throughout the growth process. The photoinduced dynamics of the newly formed I-CDW phase was probed at various stages of the growth process using a double pump scheme, where a first pump creates I-CDW domains and a second pump excites the newly formed I-CDW state. We observe larger magnitudes of the coherently excited I-CDW amplitude mode in smaller domains, which suggests that the incommensurate lattice distortion is less stable for smaller domain sizes.

cond-mat.str-el↗

arXiv2017

Coupling between a charge density wave and magnetism in an Heusler material

The Prototypical magnetic memory shape alloy Ni$_2$MnGa undergoes various phase transitions as a function of temperature, pressure, and doping. In the low-temperature phases below 260 K, an incommensurate structural modulation occurs along the [110] direction which is thought to arise from softening of a phonon mode. It is not at present clear how this phenomenon is related, if at all, to the magnetic memory effect. Here we report time-resolved measurements which track both the structural and magnetic components of the phase transition from the modulated cubic phase as it is brought into the high-symmetry phase. The results suggest that the photoinduced demagnetization modifies the Fermi surface in regions that couple strongly to the periodicity of the structural modulation through the nesting vector. The amplitude of the periodic lattice distortion, however, appears to be less affected by the demagnetizaton.

cond-mat.str-el↗

arXiv2017

Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales

Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS$_2$, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.

cond-mat.str-el↗

arXiv2016

Nonlinear electron-phonon coupling in doped manganites

We employ time-resolved resonant x-ray diffraction to study the melting of charge order and the associated insulator-metal transition in the doped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ after resonant excitation of a high-frequency infrared-active lattice mode. We find that the charge order reduces promptly and highly nonlinearly as function of excitation fluence. Density functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drive these dynamics, highlighting a new avenue of nonlinear phonon control.

cond-mat.str-el↗