arXiv subjects

D. Nicoletti

Publications and source records attributed to D. Nicoletti.

At least 19 recordsLinked to original sources

arXiv2025

Signatures of three-dimensional photo-induced superconductivity in YBa$_2$Cu$_3$O$_{6.48}$

Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of YBa$_2$Cu$_3$O$_{6+x}$, inducing a superconducting-like optical response above equilibrium $T_C$. All of the evidence collected so far has been based on the changes of the THz frequency $c$-axis response. In these measurements, the capacitive interlayer coupling was seen to transform into a superconducting-like inductive response. This assignment was strengthened by recent measurements of ultrafast magnetic field expulsion. Here, we report an experimental determination of the transient in-plane optical properties, which has so far been elusive due to the high equilibrium reflectivity and the need to evaluate minute changes in the optical response. We report the appearance of a photo-induced in-plane optical gap $2\Delta\simeq30$ cm$^{-1}$ and a divergent imaginary conductivity, both consistent with photo-induced superconductivity. A global fit to these data suggests that in- and out-of-plane electronic properties never completely equilibrate during the dynamics.

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arXiv2025

Enhanced coherent terahertz emission from critical superconducting fluctuations in YBa$_2$Cu$_3$O$_{6.6}$

Coherent terahertz (THz) emission is emerging as a powerful new tool to probe symmetry breakings in quantum materials. This method relies on second order optical nonlinearities and is complementary to second harmonic generation spectroscopy. Here, we report coherent THz emission from Josephson plasmons in underdoped YBa$_2$Cu$_3$O$_{6+x}$, and find that the amplitude of the emitted field increases dramatically close to the superconducting transition temperature, $T_C$. We show theoretically how emission is enhanced by critical superconducting fluctuations, a nonlinear analogue of critical opalescence. This observation is expected to be of general importance for the study of many thermal and quantum phase transitions.

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arXiv2023

Ultrafast Raman thermometry in driven YBa$_2$Cu$_3$O$_{6.48}$

Signatures of photo-induced superconductivity have been reported in cuprate materials subjected to a coherent phonon drive. A 'cold' superfluid was extracted from the transient Terahertz conductivity and was seen to coexist with 'hot' uncondensed quasi-particles, a hallmark of a driven-dissipative system of which the interplay between coherent and incoherent responses are not well understood. Here, time resolved spontaneous Raman scattering was used to probe the lattice temperature in the photo-induced superconducting state of YBa2Cu3O6.48. An increase in lattice temperature of approximately 80 K was observed by measuring the time dependent Raman scattering intensity of an undriven 'spectator' phonon mode. This is to be compared with an estimated increase in quasi-particle temperatures of nearly 200 K. These temperature changes provide quantitative information on the nature of the driven state and its decay, and may provide a strategy to optimize this effect.

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arXiv2023

Comment on "Light-induced melting of competing stripe orders without introducing superconductivity in La$_{1.875}$Ba$_{0.125}$CuO$_4$" (arXiv:2306.07869v1)

In the manuscript arXiv:2306.07869v1, N. L. Wang and co-authors report the results of a near-infrared pump / terahertz probe study in the stripe-ordered cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$. They measured a change in optical conductivity, but did not find signatures of transient superconductivity. From this observation they extrapolate that in all cuprates in which striped states have been excited with light, there must be no light-induced superconductivity. They conclude that "transient superconductivity cannot be induced by melting of the competing stripe orders with pump pulses whose photon energy is much higher than the superconducting gap of cuprates." Here we show that this extrapolation is unwarranted. First, the absence of light-induced superconductivity in this particular compound was already reported in a previous paper, which instead showed positive evidence for La$_{1.885}$Ba$_{0.115}$CuO$_4$. In addition, the experiment discussed here used photo-excitation with too low fluence and at a suboptimal wavelength. More broadly, a negative result in one compound is rarely compelling indication of the absence of an effect in an entire class of materials.

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arXiv2023

Comment on arXiv:2210.01114: Optical Saturation Produces Spurious Evidence for Photoinduced Superconductivity in K$_3$C$_{60}$

In the manuscript arXiv:2210.01114, Dodge and co-authors discuss the influence of pump-probe profile deformations on the reconstructed non-equilibrium optical conductivity of K$_3$C$_{60}$. They state that when pump-induced saturation of the probe response is taken into account, the reconstructed optical properties are not superconducting-like, as was claimed in a number of experimental reports by our group. We show here that the conclusion reached by Dodge et al. is unjustified. In fact, independent of the specific model, including the problematic saturation profile proposed by the authors, the reconstructed optical properties are those of a finite temperature superconductor. The true fingerprint of superconductivity, which is the $1/\omega$ divergence of the imaginary conductivity, $\sigma_{2}(\omega)$, is retained and is virtually independent of the chosen model. The only model-dependent feature is the degree of gapping in $\sigma_{1}(\omega)$. In all cases the extracted optical properties reflect the presence of residual quasiparticles, which at finite temperatures are inevitably present alongside the superfluid.

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arXiv2022

Two-fluid dynamics in driven YBa$_2$Cu$_3$O$_{6.48}$

Coherent optical excitation of certain phonon modes in YBa$_2$Cu$_3$O$_{6+x}$ has been shown to induce superconducting-like interlayer coherence at temperatures higher than $T_c$. Recent work has associated these phenomena to a parametric excitation and amplification of Josephson plasma polaritons, which are overdamped above $T_c$ but are made coherent by the phonon drive. However, the dissipative response of uncondensed quasiparticles, which do not couple in the same way to the phonon drive, has not been addressed. Here, we investigate both the enhancement of the superfluid density, $\omega\sigma_2(\omega)$, and the dissipative response of quasiparticles, $\sigma_1(\omega)$, by systematically tuning the duration and energy of the mid-infrared pulse while keeping the peak field fixed. We find that the photo-induced superfluid density saturates to the zero-temperature equilibrium value for pulses made longer than the phonon dephasing time, whilst the dissipative component continues to grow with increasing pulse duration. We show that superfluid and dissipation remain uncoupled as long as the drive is on, and identify an optimal regime of pump pulse durations for which the superconducting response is maximum and dissipation is minimized.

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arXiv2021

Coherent Emission from Surface Josephson Plasmons in Striped Cuprates

The interplay between charge order and superconductivity remains one of the central themes of research in quantum materials. In the case of cuprates, the coupling between striped charge fluctuations and local electromagnetic fields is especially important, as it affects transport properties, coherence and dimensionality of superconducting correlations. Here, we study the emission of coherent terahertz radiation in single-layer cuprates of the La$_{2-x}$Ba$_x$CuO$_4$ family, for which this effect is expected to be forbidden by symmetry. We find that emission vanishes for compounds in which the stripes are quasi-static, but is activated when $c$-axis inversion symmetry is broken by incommensurate or fluctuating charge stripes, such as in La$_{1.905}$Ba$_{0.095}$CuO$_4$ and in La$_{1.845}$Ba$_{0.155}$CuO$_4$. In this case, terahertz radiation is emitted by surface Josephson plasmons, which are generally dark modes, but couple to free space electromagnetic radiation because of the stripe modulation.

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arXiv2021

Terahertz phase slips in striped La$_{2-x}$Ba$_x$CuO$_4$

Interlayer transport in high-$T_C$ cuprates is mediated by superconducting tunneling across the CuO$_2$ planes. For this reason, the terahertz frequency optical response is dominated by one or more Josephson plasma resonances and becomes highly nonlinear at fields for which the tunneling supercurrents approach their critical value, $I_C$. These large terahertz nonlinearities are in fact a hallmark of superconducting transport. Surprisingly, however, they have been documented in La$_{2-x}$Ba$_x$CuO$_4$ also above $T_C$ for doping values near $x=1/8$, and interpreted as an indication of superfluidity in the stripe phase. Here, Electric Field Induced Second Harmonic (EFISH) is used to study the dynamics of time-dependent interlayer voltages when La$_{2-x}$Ba$_x$CuO$_4$ is driven with large-amplitude terahertz pulses, in search of other characteristic signatures of Josephson tunnelling in the normal state. We show that this method is sensitive to the voltage anomalies associated with 2$\pi$ Josephson phase slips, which near $x=1/8$ are observed both below and above $T_C$. These results document a new regime of nonlinear transport that shares features of fluctuating stripes and superconducting phase dynamics.

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arXiv2021

A phase diagram for light-induced superconductivity in $\kappa$-(ET)$_2$-X

Resonant optical excitation of certain molecular vibrations in $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br has been shown to induce transient superconducting-like optical properties at temperatures far above equilibrium $T_c$. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl and the metallic compound $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$. We find non-equilibrium photoinduced superconductivity only in $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are pre-requisites for this effect.

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arXiv2020

Dynamical order and superconductivity in a frustrated many-body system

In triangular lattice structures, spatial anisotropy and frustration can lead to rich equilibrium phase diagrams with regions containing complex, highly entangled states of matter. In this work we study the driven two-rung triangular Hubbard model and evolve these states out of equilibrium, observing how the interplay between the driving and the initial state unexpectedly shuts down the particle-hole excitation pathway. This restriction, which symmetry arguments fail to predict, dictates the transient dynamics of the system, causing the available particle-hole degrees of freedom to manifest uniform long-range order. We discuss implications of our results for a recent experiment on photo-induced superconductivity in ${\rm \kappa - (BEDT-TTF)_{2}Cu[N(CN)_{2}]Br}$ molecules.

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arXiv2020

Photo-molecular high temperature superconductivity

Superconductivity in organic conductors is often tuned by the application of chemical or external pressure. With this type of tuning, orbital overlaps and electronic bandwidths are manipulated, whilst the properties of the molecular building blocks remain virtually unperturbed.Here, we show that the excitation of local molecular vibrations in the charge-transfer salt $\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br$ induces a colossal increase in carrier mobility and the opening of a superconducting-like optical gap. Both features track the density of quasi-particles of the equilibrium metal, and can be achieved up to a characteristic coherence temperature $T^* \approxeq 50 K$, far higher than the equilibrium transition temperature $T_C = 12.5 K$. Notably, the large optical gap achieved by photo-excitation is not observed in the equilibrium superconductor, pointing to a light induced state that is different from that obtained by cooling. First-principle calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photo-molecular superconductivity.

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arXiv2019

Pump frequency resonances for light-induced incipient superconductivity in YBa$_2$Cu$_3$O$_{6.5}$

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature, $T_C$, as evidenced by the terahertz frequency optical properties in the non-equilibrium state. In YBa$_2$Cu$_3$O$_{6+x}$ this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like La$_{2-x}$Ba$_x$CuO$_4$, similar effects were reported also for excitation at near infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency dependent photo-susceptibility for this phenomenon. Here, we make use of a newly developed optical parametric amplifier, which generates widely tunable high intensity femtosecond pulses, to excite YBa$_2$Cu$_3$O$_{6.5}$ throughout the entire optical spectrum (3 - 750 THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25 - 750 THz), a second resonance is observed around the charge transfer band edge at ~350 THz. These observations highlight the importance of coupling to the electronic structure of the CuO$_2$ planes, either mediated by a phonon or by charge transfer.

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arXiv2018

Magnetic-Field Tuning of Light-Induced Superconductivity in Striped La$_{2-x}$Ba$_x$CuO$_4$

Optical excitation of stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ has been shown to transiently enhance superconducting tunneling between the CuO$_2$ planes. This effect was revealed by a blue-shift, or by the appearance of a Josephson Plasma Resonance in the terahertz-frequency optical properties. Here, we show that this photo-induced state can be strengthened by the application of high external magnetic fields oriented along the c-axis. For a 7-Tesla field, we observe up to a ten-fold enhancement in the transient interlayer phase correlation length, accompanied by a two-fold increase in the relaxation time of the photo-induced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically-enhanced interlayer coupling in La$_{2-x}$Ba$_x$CuO$_4$ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photo-induced state may emerge from activated tunneling between optically-excited stripes in adjacent planes.

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arXiv2018

Pressure tuning of light-induced superconductivity in K3C60

Optical excitation at terahertz frequencies has emerged as an effective means to manipulate complex solids dynamically. In the molecular solid K3C60, coherent excitation of intramolecular vibrations was shown to transform the high temperature metal into a non-equilibrium state with the optical conductivity of a superconductor. Here we tune this effect with hydrostatic pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth is detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesised previously, and rather extends all the way to room temperature.

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arXiv2017

Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3

Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle inter-relation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights, by tracking the relaxation kinetics following excitation at frequencies related to the broken symmetry state. Here, we investigate the transient terahertz conductivity of BaPb1-xBixO3 - a material for which superconductivity is adjacent to a competing CDW phase - after optical excitation tuned to the CDW absorption band. In insulating BaBiO3 we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaPb0.72Bi0.28O3 (superconducting below Tc=7K), a similar response was also found immediately above Tc. This observation evidences the presence of a robust gap up to T=40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behaviour was observed in the same material fo T>40 K. Here, the photo-conductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above Tc in underdoped BaPb1-xBixO3, and provide new information on the dynamical interplay between superconductivity and charge order.

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arXiv2017

Optical melting of the transverse Josephson plasmon: a comparison between bilayer and trilayer cuprates

We report on an investigation of the redistribution of interlayer coherence in the trilayer cuprate Bi2Sr2Ca2Cu3O10. The experiment is performed under the same apical-oxygen phonon excitation discussed in the past for the bilayer cuprate YBa2Cu3O6.5. In Bi2Sr2Ca2Cu3O10, we observe a similar spectral weight loss at the transverse plasma mode resonance seen in YBa2Cu3O6.5. However, this feature is not accompanied by light-enhanced interlayer coherence that was seen in YBa2Cu3O6+x, for which the transverse plasma mode is observed at equilibrium even in the normal state. These new observations offer new experimental perspective in the context of the physics of light-enhanced interlayer coupling in various cuprates.

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arXiv2016

Dynamical decoherence of the light induced interlayer coupling in YBa$_{2}$Cu$_{3}$O$_{6+δ}$

Optical excitation of apical oxygen vibrations in YBa$_{2}$Cu$_{3}$O$_{6+δ}$ has been shown to enhance its c-axis superconducting-phase rigidity, as evidenced by a transient blue shift of the equilibrium inter-bilayer Josephson plasma resonance. Surprisingly, a transient c-axis plasma mode could also be induced above T$_{c}$ by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of YBa$_{2}$Cu$_{3}$O$_{6+δ}$. However, despite the similarities between the above T$_{c}$ transient plasma mode and the equilibrium Josephson plasmon, alternative explanations involving high mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light-induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport, and rather support transient superfluid tunneling as the origin of the light-induced interlayer coupling in YBa$_{2}$Cu$_{3}$O$_{6+δ}$.

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arXiv2016

Restoring Interlayer Josephson Coupling in La_{1.885}Ba_{0.115}CuO_4 by Charge Transfer Melting of Stripe Order

We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La_{1.885}Ba_{0.115}CuO_4 (LBCO). Time-Resolved Resonant Soft X-Ray Diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson Plasma Resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of about 0.5 mJ/cm2. Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order and rearrangement of the crystal structure.

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