arXiv subjects

H. P. Ojeda Collado

Publications and source records attributed to H. P. Ojeda Collado.

15 recordsLinked to original sources

arXiv2025

Fluctuation amplification engineering in multimode Raman-cavity systems

Parametric amplification is a key ingredient of a wide range of phenomena, from the classical to the quantum domain. Although such phenomena have been demonstrated in non-equilibrium settings, their use for fluctuation engineering has been put forth in Raman-cavity hybrids only recently. In this work, we generalize fluctuation engineering to a multi-mode scenario in which multiple Raman-active modes interact nonlinearly with multiple cavity modes. We demonstrate the emergence of resonant and non-resonant collective fluctuations that can be non-reciprocally controlled by engineering the band dispersion of photons and phonons. As an example we show how Raman fluctuations can be selectively attenuated by tuning the photonic bandgap or even nonresonantly amplified, in marked contrast to the single-mode scenario. We also identify a regime in which the amplification of cavity fluctuations in a specific mode is boosted, surpassing a $\sqrt{N}$ scaling with increasing number of $N$ Raman and cavity modes. Our study reveals the key role of multi-mode interactions on fluctuations in nonlinear cavity-matter hybrids. Noise engineering through different photon and phonon dispersions, as demonstrated here, could be leveraged for the design of novel quantum sensing platforms and advanced spectroscopy in the THz regime.

quant-ph↗

arXiv2025

Coherent Optical Control of Electron Dynamics in Patterned Graphene Nanoribbons

The field of coherent electronics aims to advance electronic functionalities by utilizing quantum coherence. Here, we demonstrate a viable and versatile methodology for controlling electron dynamics optically in graphene nanoribbons. In particular, we propose to flatten the band structure of armchair graphene nanoribbons via control electrodes, arranged periodically along the extended direction of the nanoribbon. This addresses a key mechanism for dephasing in solids, which derives from the momentum dependence of the energy gap between the valence and the conduction band. We design an optimal driving field pulse to produce collective Rabi oscillations between these bands, in their flattened configuration. As an example for coherent control, we show that these optimized pulses can be used to invert the entire electronic band population by a $π$ pulse in a reversible fashion, and to create a superposition state via a $π/2$ pulse, which generates an alternating photocurrent. Our proposal consists of a platform and methodological approach to optically control the electron dynamics of graphene nanoribbons, paving the way toward novel coherent electronic and quantum information processing devices in solid-state materials.

cond-mat.mes-hall↗

arXiv2025

Hybridization of the amplitude mode in a confined fermionic superfluid

In phase transitions, spontaneous symmetry breaking results in a non-zero order parameter and two collective excitations: the Goldstone and the amplitude mode. These modes, which define key properties of superconductors and fermionic superfluids, are well-understood in homogeneous systems. However, their behavior under strong confinement remains largely unexplored, particularly when their excitation energy becomes comparable to the imposed discrete level spacing. In this scenario, hybridization between different collective modes is expected to take place. Here, we show how the amplitude mode hybridizes with a spatial mode in a confined fermionic superfluid. Using lattice modulation spectroscopy, we observe the evolution of the mode throughout the entire crossover from the Bardeen-Cooper-Schrieffer (BCS) state to Bose-Einstein condensation (BEC) of molecules. In the BCS regime, the excitation energy is located at twice the pairing gap, then gradually becomes an in-gap excitation in the strongly correlated regime. Further to the BEC limit, the excitation energy approaches twice the level spacing. The spectral weight of this mode vanishes when approaching the superfluid critical temperature. Our experimental results are in excellent agreement with an effective field theory, providing strong evidence that amplitude oscillations hybridize with and eventually transform into breathing oscillations of the order parameter. The strong modification of the excitation spectrum reveals how confinement and finite-size effects impact fundamental modes of symmetry-broken states.

cond-mat.quant-gas↗

arXiv2024

Coherent control of photoconductivity in graphene nanoribbons

We study the photoconductivity response of graphene nanoribbons with armchair edges in the presence of dissipation using a Lindblad-von Neumann master equation formalism. We propose to control the transport properties by illuminating the system with light that is linearly polarized along the finite direction of the nanoribbon while probing along the extended direction. We demonstrate that the largest steady-state photocurrent occurs for a driving frequency that is slightly blue-detuned to the electronic band gap proportional to the width of the nanoribbon. We compare the photoconductivity in the presence of coherent and incoherent light and conclude that the enhancement of the photoconductivity for blue-detuned driving relies on the coherence of the driving term. Based on this result we propose a switching protocol for fast control of the photocurrent on a time scale of a few picoseconds. Furthermore, we suggest a design for a heterostructure of a graphene nanoribbon and a high-Tc superconductor, that is operated as a transistor as a step towards next-generation coherent electronics.

cond-mat.mes-hall↗

arXiv2024

Equilibrium Parametric Amplification in Raman-Cavity Hybrids

Parametric resonances and amplification have led to extraordinary photoinduced phenomena in pump-probe experiments. While these phenomena manifest themselves in out-of-equilibrium settings, here, we present the striking result of parametric amplification in equilibrium. In particular, we demonstrate that quantum and thermal fluctuations of a Raman-active mode amplifies light inside a cavity, at equilibrium, when the Raman mode frequency is twice the cavity mode frequency. This noise-driven amplification leads to the creation of an unusual parametric Raman polariton, intertwining the Raman mode with cavity squeezing fluctuations, with smoking gun signatures in Raman spectroscopy. In the resonant regime, we show the emergence of not only quantum light amplification but also localization and static shift of the Raman mode. Apart from the fundamental interest of equilibrium parametric amplification our study suggests a resonant mechanism for controlling Raman modes and thus matter properties by cavity fluctuations. We conclude by outlining how to compute the Raman-cavity coupling, and suggest possible experimental realization

quant-ph↗

arXiv2022

Dynamical phases transitions in periodically driven Bardeen-Cooper-Schrieffer systems

We present a systematic study of the dynamical phase diagram of a periodically driven BCS system as a function of drive strength and frequency. Three different driving mechanism are considered and compared: oscillating density of states, oscillating pairing interaction and oscillating external paring field. We identify the locus in parameter space of parametric resonances and four dynamical phases: Rabi-Higgs, gapless, synchronized Higgs and time-crystal. We demonstrate that the main features of the phase diagram are quite robust to different driving protocols and discuss the order of the transitions. By mapping the BCS problem to a collection of nonlinear and interacting classical oscillators, we shed light on the origin of time-crsytalline phases and parametric resonances appearing for subgap excitations.

cond-mat.supr-con↗

arXiv2022

Engineering Higgs dynamics by spectral singularities

We generalize the dynamical phase diagram of a Bardeen-Cooper-Schrieffer condensate, considering attractive to repulsive, i.e., critical quenches (CQ) and a non-constant density of states (DOS). We show that different synchronized Higgs dynamical phases can be stabilized, associated with singularities in the density of states (DOS) and different quench protocols. In particular, the CQ can stabilize an overlooked high-frequency Higgs dynamical phase related to the upper edge of the fermionic band. For a compensated Dirac system we find a Dirac-Higgs mode associated with the cusp singularity at the Fermi level, and we show that synchronized phases become more pervasive across the phase diagram. The relevance of these remarkable phenomena and their realization in ensembles of fermionic cold atoms confined in optical lattices is also discussed.

cond-mat.quant-gas↗

arXiv2021

Emergent parametric resonances and time-crystal phases in driven BCS systems

We study the out-of-equilibrium dynamics of a Bardeen-Cooper-Schrieffer condensate subject to a periodic drive. We demonstrate that the combined effect of drive and interactions results in emerging parametric resonances, analogous to a vertically driving pendulum. In particular, Arnold tongues appear when the driving frequency matches $2Δ_0/n$, with $n$ a natural number, and $Δ_0$ the equilibrium gap parameter. Inside the Arnold tongues we find a commensurate time-crystal condensate which retains the $U(1)$ symmetry breaking of the parent superfluid/superconducting phase and shows an additional time-translational symmetry breaking. Outside these tongues, the synchronized collective Higgs mode found in quench protocols is stabilized without the need of a strong perturbation. Our results are directly relevant to cold-atom and condensed-matter systems and do not require very long energy relaxation times to be observed.

cond-mat.supr-con↗

arXiv2019

Spectral fingerprints of the non-linear dynamics of driven superconductors with dissipation

We study the out-of-equilibrium dynamics of a BCS superconductor in the presence of a periodic drive with a frequency $ω_d$ larger than the equilibrium superconducting gap and an external bath providing dissipation. Similar to the dissipationless case, a subset of quasiparticles, resonant with the drive, synchronizes and produces interesting non-linear phenomena. For small dissipation Rabi-Higgs oscillations [Collado et al. Phys. Rev. B 98, 214519 (2018)] can be observed as a transient effect. At long-times in contrast, dissipation leads to a steady state with a population imbalance that increases quadratically for small drives and saturates for large drives as in non-linear quantum optics. We show also that second harmonic generation is allowed for a drive which acts on the BCS coupling constant. We also compute the intensity of time- and angle-resolved photo-emission spectroscopy (tr-ARPES) and time-resolved tunneling spectra. The excited quasiparticle population appears as a decrease (increase) in the photo-emission intensity at energy $-ω_d/2$ ($+ω_d/2$) measured from the chemical potential. The tunneling intensity shows a time-dependent depression at $\pmω_d/2$ due to the lacking of spectral density and population unbalance causing Pauli blockade. We predict that at short times, compared to the energy relaxation time, both experiments will show oscillations with the Rabi-Higgs frequency.

cond-mat.supr-con↗

arXiv2019

Impurity states and Localization in Bilayer Graphene: the Low Impurity Concentration Regime

We study the problem of non-magnetic impurities adsorbed on bilayer graphene in the diluted regime. We analyze the impurity spectral densities for various concentrations and gate fields. We also analyze the effect of the adsorbate on the local density of states (LDOS) of the different C atoms in the structure and present some evidence of strong localization for the electronic states with energies close to the Dirac point.

cond-mat.mes-hall↗

arXiv2019

Fate of dynamical phases of a BCS superconductor beyond the dissipationless regimen

The BCS model of an isolated superconductor initially prepared in a nonequilibrium state, predicts the existence of interesting dynamical phenomena in the time-dependent order parameter as decaying oscillations, persistent oscillations and overdamped dynamics. To make contact with real systems remains an open challenge as one needs to introduce dissipation due to the environment in a self-consistent computation. Here, we reach this goal with the use of the Keldysh formalism to treat the effect of a thermal bath. We show that, contrary to the dissipationless case, all dynamical phases reach the equilibrium order parameter in a characteristic time that depends on the coupling with the bath. Remarkably, as time evolves, the overdamped phase shows a fast crossover where the superconducting order parameter recovers to reach a state with a well-developed long range order that tends towards equilibrium with the damped Higgs mode oscillations. Our results provide a benchmark for the description of the dynamics of real out-of-equilibrium superconductors relevant for quantum technological applications.

cond-mat.supr-con↗

arXiv2018

Population inversion and dynamical phase transitions in a driven superconductor

We consider a superconductor in which the density of states at the Fermi level or the pairing interaction is driven periodically with a frequency larger than the superconducting gap in the collisionless regime. We show by numerical and analytical computations that a subset of quasiparticle excitations enter into resonance and perform synchronous Rabi oscillations leading to cyclic population inversion with a frequency that depends on the amplitude of the drive. As a consequence a new "Rabi-Higgs" mode emerges. Turning off the drive at different times and modulating the strength allows access to all known dynamical phases of the order parameter: persistent oscillations, oscillations with damping and overdamped dynamics. We discuss physical realizations of the drive and methods to detect the dynamics.

cond-mat.supr-con↗

arXiv2016

Experimental approval of the extended flat bands and gapped subbands in rhombohedral multilayer graphene

Graphene layers are known to stack in two stable configurations, namely ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report the first magneto optical study of a large ABC domain in a graphene multilayers flake, with ABC sequences exceeding 17 graphene sheets. The ABC-stacked multilayers can be fingerprinted with a characteristic electronic Raman scattering response, which persists even at room temperatures. Tracing the magnetic field evolution of the inter Landau level excitations from this domain gives strong evidence to the existence of a dispersionless electronic band near the Fermi level, characteristic of such stacking. Our findings present a simple yet powerful approach to probe ABC stacking in graphene multilayer flakes, where this highly degenerated band appears as an appealing candidate to host strongly correlated states.

cond-mat.mes-hall↗

arXiv2016

Bilayer graphene under pressure: Electron-hole Symmetry Breaking, Valley Hall Effect, and Landau Levels

The electronic structure of bilayer graphene under pressure develops very interesting features with an enhancement of the trigonal warping and a splitting of the parabolic touching bands at the K point of the reciprocal space into four Dirac cones, one at K and three along the T symmetry lines. As pressure is increased, these cones separate in reciprocal space and in energy, breaking the electron-hole symmetry. Due to their energy separation, their opposite Berry curvature can be observed in valley Hall effect experiments and in the structure of the Landau levels. Based on the electronic structure obtained by Density Functional Theory, we develop a low energy Hamiltonian that describes the effects of pressure on measurable quantities such as the Hall conductivity and the Landau levels of the system.

cond-mat.mtrl-sci↗

arXiv2015

Impurities and electronic localization in graphene bilayers

We analyze the electronic properties of bilayer graphene with Bernal stacking and a low concentration of adatoms. Assuming that the host bilayer lies on top of a substrate, we consider the case where impurities are adsorbed only on the upper layer. We describe non-magnetic impurities as a single orbital hybridized with carbon's pz states. The effect of impurity doping on the local density of states with and without a gated electric field perpendicular to the layers is analyzed. We look for Anderson localization in the different regimes and estimate the localization length. In the biased system, the field induced gap is partially filled by strongly localized impurity states. Interestingly, the structure, distribution and localization length of these states depend on the field polarization.

cond-mat.mes-hall↗