Energy Relaxation Research Articles

What Can we Learn from Nonequilibrium Response of a Strange Metal?

We critically address the recent experiment by L. Chen et al. [Science 382, 907 (2023)] on nonequilibrium transport and noise in a strange metal YbRh2Si2 patterned into the nanowire shape. In the long device, resistivity, differential resistance and current noise data seem to be consistent allowing us to extract electron–phonon coupling and the temperature dependence of electron–phonon scattering length. The obtained values can be reconciled with the experimental data for the short device only assuming the significant contact resistance. We discuss its possible origin as due to the current redistribution between YbRh2Si2 and its gold covering, and reveal that this redistribution contact resistance should be proportional to the YbRh2Si2 resistivity. We also discuss some subtleties of the noise measurements. Overall, neglecting electron–phonon energy relaxation even in the shortest devices is arguable so that the observed shot noise suppression can hardly be attributed to the failure of quasiparticle concept.

Single-shot readout of a superconducting qubit using a thermal detector

Measuring the state of a qubit is a key fundamental operation of a quantum computer. High-fidelity single-shot readout of superconducting qubits can be achieved using parametric amplifiers at millikelvin temperatures. However, scaling parametric amplifiers beyond hundreds of qubits is challenging due to practical size and power limitations. Nanobolometers can, in contrast, offer scalability, sensitivity and speed suitable for qubit readout. Here we show that a bolometer can provide single-shot qubit readout with a readout duration of 13.9 μs and a single-shot fidelity of 0.618. The fidelity is mainly limited by the energy relaxation time of the qubit (28 μs), and a fidelity of 0.927 is found after removing errors arising from this relaxation. In the future, higher-fidelity single-shot readout may be achieved through improvements in chip design and experimental setup, as well as a change in the bolometer absorber material to reduce the readout time to the level of hundreds of nanoseconds and below.

Microwave assisted self-repairable vitrimeric coating for anti-corrosive applications

Vitrimeric coatings are revolutionary and most widely emerging materials for metal corrosion inhibition. As harsh environmental conditions damages polymer-derived protective films and results into micro cracks and corroded metal surfaces. In this work, an active microwave responsive vitrimeric epoxy coating embedded with weathering resistant properties is introduced along with systematic comparative study for the effect of ortho and para position of amine in hardening agent. The developed microwave radiation responsive coating system felicitates self-healing due to disulfide exchange mechanism and corrosion inhibition properties in marine environments. Compared to conventional coatings, prepared graphene oxide (GO) incorporated vitrimeric network showed more promising protection against corrosion in offshore applications with 95.4 % protection efficiency and hydrophobicity with 96-degree contact angle. This study covers the self-healing properties, thermal mechanical study, hydrophobicity measurement, and thorough corrosion examination of synthesized material coated on 304 grade SS substrate. The coating reports excellent self-healing efficiency, reliable mechanical strength (storage modulus-129GPa and loss modulus-83GPa), fast stress relaxation and low activation energy (63KJ/mol) for dynamic disulfide exchange reaction, furthermore low corrosion rate of 1.20 × 10−3mm/y was observed for the coating. Thus, demonstrates its importance for self-healable anti-corrosive application for the marine environment.

Hypersonic boost-glide systems: Flight mechanics and plasma parameters evaluation through aero-thermo-chemical computational fluid dynamics

Currently, the conception, design, and development of hypersonic space-borne systems is a central topic of discussion among researchers and industrial stakeholders. In the design phase of a hypersonic manoeuvrable vehicle, it is essential to characterise the thermal heating and manage the problem of the communication blackout, in addition to other challenges concerning guidance, navigation and control, and propulsion. The interference of electromagnetic waves propagating in plasma affects the datalink and the radar detection of hypersonic targets. This work aims to provide valuable information about the aero-thermo-chemical behaviour of the fluid in the region around a hypersonic glider. Launch and re-entry phases of boost-glide systems have been modelled to perform trajectory predictions and assess range and flight paths under the effect of different manoeuvres, defining their possible performances. The resulting flight conditions are used to study the flow field surrounding a glider geometry similar to the Hypersonic Technology Vehicle-2. The thermodynamic and chemical fields around the body have been determined, and plasma parameters for novel electromagnetic interaction studies have been evaluated. Computational fluid dynamics simulations have been carried out to solve the averaged Navier-Stokes equations with the k−ω model for turbulent flows, coupled with the energy equation. Chemical non-equilibrium phenomena involving electronic energy relaxation as well as ionisation reactions have also been included. A gas mixture potentially composed of 11 species has been considered, namely oxygen and nitrogen derivatives. The electron number density is computed to evaluate the electron plasma and collision frequencies, essential parameters to characterise the electromagnetic interference caused by the plasma sheath. Among other results, high temperatures are reached at the nose region of the body in specific conditions of velocity and altitude, and non-negligible dissociation or ionisation of chemical species occurs. Plasma frequencies up to tens of GHz are reached in a region close to the body surface, while lower interference characterises the extended ionised wake.

Raman time-delay in attosecond transient absorption of strong-field created krypton vacancy

Strong field ionization injects a transient vacancy in the atom which is entangled to the outgoing photoelectron. When the electron is finally detached, the ion is populated at different excited states with part of coherence information lost. The preserved coherence of matter after interacting with intense short pulses has important consequences on the subsequent nonequilibrium evolution and energy relaxation. Here we employ attosecond transient absorption spectroscopy to measure the time-delay of resonant transitions of krypton vacancy during their creation. We have observed that the absorptions by the two spin-orbit split states are modulated at different paces when varying the time-delay between the near-infrared pumping pulse and the attosecond probing pulse. It is shown that the coupling of the ions with the remaining field leads to a suppression of ionic coherence. Comparison between theory and experiments uncovers that coherent Raman coupling induces time-delay between the resonant absorptions, which provides insight into laser-ion interactions enriching attosecond chronoscopy.

2D Time-Domain Spectroscopy for Determination of Energy and Momentum Relaxation Rates of Hydrogen-Like Donor States in Germanium.

We present measurements of the coherence times of excited states of hydrogen-like arsenic impurities in germanium (Ge:As) using a table-top two-dimensional time-domain spectroscopy (2D-TDS) system. We show that this laboratory system is capable of resolving the coherence lifetimes of atomic-like excited levels of impurity centers in semiconductors, such as those used in solid-state quantum information technologies, on a subpicosecond time scale. By fitting the coherent nonlinear response of the system with the known intracenter transition frequencies, we are able to monitor coherent population transfer and decay of the transitions from the 2p0 and 2p± states for different low excitation pulse fields. Furthermore, by examining the off-diagonal resonances in the 2D frequency-domain map, we are able to identify coherences between excited electronic states that are not visible via conventional single-frequency pump-probe or Hahn-echo measurements.

Phonon-mediated ultrafast energy- and momentum-resolved hole dynamics in monolayer black phosphorus.

The electron-phonon scattering plays a crucial role in determining the electronic, transport, optical, and thermal properties of materials. Here, we employ a non-Markovian stochastic Schrödinger equation (NMSSE) in momentum space, together with abinitio calculations for energy bands and electron-phonon interactions, to reveal the phonon-mediated ultrafast hole relaxation dynamics in the valence bands of monolayer black phosphorus. Our numerical simulations show that the hole can initially remain in the high-energy valence bands for more than 100fs due to the weak interband scatterings, and its energy relaxation follows single-exponential decay toward the valence band maximum after scattering into low-energy valence bands. The total relaxation time of holes is much longer than that of electrons in the conduction band. This suggests that harnessing the excess energy of holes may be more effective than that of electrons. Compared to the semiclassical Boltzmann equation based on a hopping model, the NMSSE highlights the persistence of quantum coherence for a long time, which significantly impacts the relaxation dynamics. These findings complement the understanding of hot carrier relaxation dynamics in two-dimensional materials and may offer novel insights into harnessing hole energy in photocatalysis.

Can classical mechanics sense conical intersection?

Conical intersection (CI) leads to fast electronic energy transfer. However, Hamm and Stock [Phys. Rev. Lett. 109, 173201 (2012)] showed the existence of a vibrational CI and its role in vibrational energy relaxation. In this paper, we further investigate the vibrational energy relaxation using an isolated model Hamiltonian system of four vibrational modes with two distinctively different timescales (two fast modes and two slow modes). We show that the excitation of the slow modes plays a crucial role in the energy relaxation mechanism. We also analyze the system from a mixed quantum-classical (surface hopping method) and a completely classical point of view. Notably, surface hopping and even classical simulations also capture fast energy relaxation, which is a signature of CI's existence.

On the Statistical Regime, Coherence versus Incoherence and Ergodicity of Quantum Vibrational Trajectories in Soft Condensed Molecular Systems.

A theoretical-computational procedure, recently proposed for modelling Vibrational Energy Relaxation (VER) processes of a molecule (Quantum Center, QC) embedded in a complex atomic-molecular system, is extended and applied for analyzing in detail the features of the QC density matrix (DM) temporal evolution. The results, obtained using aqueous azide ion as a case study, show the total lack of coherence in the DM, when the system is prepared to be initially in a pure vibrational eigenstate. This finding is fully in line with the statistical interpretation of the process typically adopted also in the experimental studies where the relaxation processes are all described within the typical schemes of chemical kinetics. Consistently, when the initial vibrational state corresponds to an eigenstate mixture, although initially coherent, the DM relaxes to a fully incoherent condition with a mean lifetime related to the one of the diagonal elements relaxation. These specific DM features turn out to be essentially governed by the thermal equilibrium condition of the atomic-molecular classical coordinates which drive the ensemble of the quantum-trajectories toward the observed statistical regime. Finally, from the analysis of a single long timescale quantum vibrational trajectory it also clearly emerges its ergodic behaviour.

Vibrational anharmonicity of A-band related optical center and its temperature dependence studied by femtosecond laser excitation in bulk natural diamond

Unusual A-band spectra with well-resolved quasi-periodical phonon progressions of multiple peaks (N\(\sim \)9) were excited in a natural diamond by 515-nm, 300-fs laser pulses at variable pre-heating temperatures of 24-200oC. The non-radiative multi-phonon part of the relaxation path was comparable with the total relaxation energy (zero-phonon line energy), pointing out some extended defects (e.g., dislocations) as the recombination center of the A-band emission. The derived temperature dependences of the peak intensities, separations and half-widths of the separate spectral peaks in the A-band phonon progressions indicate the different trends for vibration-free zero-phonon transition and vibration-related lower-energy transitions to high vibrational levels of the bottom state – lower thermal damping and more softened phonon for the zero-phonon transition, also implying the extended defect origin of the A-band photoluminescence.

Excitation energy transfer and vibronic relaxation through light-harvesting dendrimer building blocks: A nonadiabatic perspective.

The light-harvesting excitonic properties of poly(phenylene ethynylene) (PPE) extended dendrimers (tree-like π-conjugated macromolecules) involve a directional cascade of local excitation energy transfer (EET) processes occurring from the "leaves" (shortest branches) to the "trunk" (longest branch), which can be viewed from a vibronic perspective as a sequence of internal conversions occurring among a connected graph of nonadiabatically coupled locally excited electronic states via conical intersections. The smallest PPE building block that is able to exhibit EET, the asymmetrically meta-substituted PPE oligomer with one acetylenic bond on one side and two parallel ones on the other side (hence, 2-ring and 3-ring para-substituted pseudo-fragments), is a prototype and the focus of the present work. From linear-response time-dependent density functional theory electronic-structure calculations of the molecule as regards its first two nonadiabatically coupled, optically active, singlet excited states, we built a (1 + 2)-state-8-dimensional vibronic-coupling Hamiltonian model for running subsequent multiconfiguration time-dependent Hartree wavepacket relaxations and propagations, yielding both steady-state absorption and emission spectra as well as real-time dynamics. The EET process from the shortest branch to the longest one occurs quite efficiently (about 80% quantum yield) within the first 25fs after light excitation and is mediated vibrationally through acetylenic and quinoidal bond-stretching modes together with a particular role given to the central-ring anti-quinoidal rock-bending mode. Electronic and vibrational energy relaxations, together with redistributions of quantum populations and coherences, are interpreted herein through the lens of a nonadiabatic perspective, showing some interesting segregation among the foremost photoactive degrees of freedom as regards spectroscopy and reactivity.

Ultrafast All‐Optical Control of Light Chirality with Nanostructured Graphene

AbstractUltrafast nanophotonics is a rapidly growing area of study focused on creating nanodevices that can modulate the properties of light at, to this date, unparalleled speed. To facilitate the growth of this field, there is a growing need for compact metamaterial designs for the manipulation of the amplitude, phase, and polarization of light. One promising strategy involves leveraging the optical nonlinearity of nanostructured materials to alter their permittivity by interacting with high‐intensity ultrashort laser pulses. This study showcases how such requirements can be met through the utilization of 2D materials, particularly graphene. The nonlinear optical response of a graphene nanorectangle array is theoretically modeled to achieve all‐optical, fully reversible, broadband, and ultrafast dynamic control of light chirality. This is achieved by taking advantage of the energy relaxation dynamics of coherently excited localized plasmons supported by the metasurface, and the transient increase in electron temperature in graphene. Using finite‐difference time‐domain simulations, ultrafast dynamic tuning between circular and linearly polarized light is demonstrated. The proposed platform gives promise for ultrathin, CMOS‐compatible nanophotonic systems that can provide high‐speed, room‐temperature modulation of light polarization.

Energy relaxation in a vacuum capacitor-resistor circuit: measurement of multiple decays with divergent time constants

The decay of the electrical energy in a resistor-vacuum capacitor circuit is shown to involve multiple relaxation processes, with dramatically different time constants. This is measured using a vacuum capacitor to eliminate the effect of a dielectric between the plates (polypropylene capacitors are shown to exhibit similar behavior). A simple phenomenological model accounts for this behavior in spite of the difficulty in applying Maxwell's equations to such a circuit. These results will lead to a revision of our understanding of the physics of circuits, having particular impact on applications that use capacitors as sensors in collecting precision data (such as found in quantum measurements and dielectric spectroscopy).

Phonon heat removal from metal nanoparticles and dynamics of nanoparticle cooling at low temperatures

The theoretical analysis of the energy relaxation of an electron–phonon system of metal nanoparticles embedded in a dielectric matrix is usually based on semiphenomenological dynamic equations for electron and phonon temperatures (two-temperature model), which does not take into account the nonthermal nature of the phonon distribution function. In this work, we use a microscopic model that describes the dynamics of the electron–phonon system of metal nanorods and metal spherical nanoparticles in terms of the kinetic equation for the phonon distribution function. We focus on the size effect in the transfer of heat from a nanoparticle to a dielectric matrix. If the dimensions of the nanoparticle are much larger than the phonon-electron mean free path, then the heat transfer is determined by the properties of the interface between the nanoparticle and the matrix. In the opposite case, heat removal is determined solely by the parameters of the electron–phonon interaction in a metal nanoparticle. The dynamics of cooling of nanoparticles is also considered and the dependence of the electron temperature on time is obtained.

High-Pressure Phases of Boron Pnictides BX (X = As, Sb, Bi) with Quartz Topology from First Principles

The superdense hexagonal boron pnictides BX (X = As, Sb, Bi), whose structures are formed by distorted tetrahedra and characterized by a quartz-derived (qtz) topology, have been predicted from first principles as potential high-pressure phases. From full geometry structure relaxation and ground state energy calculations based on quantum density functional theory (DFT), qtz BX was found to be mechanically (elastic constants) and dynamically (phonons) stable. From the energy–volume equations of state, at high but experimentally accessible pressures, qtz boron pnictides were found to be more energetically favorable than corresponding cubic zinc–blende phases with diamond-like (dia) topology. According to the electronic band structures, the zinc–blende BX have larger band gaps than the qtz phases, which can be attributed to the higher covalence of the latter. A metallic behavior is only observed for qtz BBi, which is related to the dynamic instability as it follows from the phonon band structure.

Theoretical study on the ESIPT and photophysical properties for molecules with different extension π conjugation

In a recent study, Jincheng Si and coworkers designed four candidate linker units with different extension π conjugation sizes and directions. These molecules were used for the synthesis of metal–organic framework (MOF) structures known as Olympic rings, which can detect the Cr2O72- in the aqueous phase. However, the experimental work did not provide a qualitative understanding of the excited-state dynamic mechanism. Therefore, this research focused on investigating the excited state intramolecular proton transfer (ESIPT) process and the photophysical properties of these molecules. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were employed to conduct an in-depth analysis. The molecule structure properties were investigated by analyzing the hydrogen bond parameters and the infrared (IR) vibrational spectra. The calculated absorption and emission spectra were in good agreement with the experimental values, it was proved that the correctness of the optimized structure and the rationality of our calculation method, and we can judge their detection sensitivity by the sizes of the Stokes shift. The Hirshfeld method was used to study the enhancement mechanism of hydrogen bond interaction. Furthermore, the reaction mechanism and ESIPT reactivity of molecules with different extension π conjugation sizes and directions were explained by calculating and plotting the relaxed potential energy curve, electrostatic potentials (ESPs) and frontier molecular orbitals (FMOs). Our research found that extension π conjugation in different sizes and directions can either promote or inhibit the occurrence of the ESIPT reactions and the characteristic of intramolecular charge transfer (ICT), thereby altering the fluorescence performance. Based on our findings, we concluded that the ESIPT activity and photophysical properties of the H4BIHQC were higher than the other three molecules. This discovery enhances the potential application value of MOF as a fluorescence detection tool.

Relaxation terms for anomalous hydrodynamic transport in Weyl semimetals from kinetic theory

We consider as a model of Weyl semimetal thermoelectric transport a (3 + 1)-dimensional charged, relativistic and relaxed fluid with a U(1)V × U(1)A chiral anomaly. We take into account all possible mixed energy, momentum, electric and chiral charge relaxations, and discover which are compatible with electric charge conservation, Onsager reciprocity and a finite DC conductivity. We find that all relaxations respecting these constraints necessarily render the system open and violate the second law of thermodynamics. We then demonstrate how the relaxations we have found arise from kinetic theory and a modified relaxation time approximation. Our results lead to DC conductivities that differ from those found in the literature opening the path to experimental verification.

Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure.

In two-dimensional semiconductors, cooperative and correlated interactions determine the material's excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlated exciton state formed by the Coulomb interaction between single-particle holes and electrons. We find that the ultrafast transfer of an exciton's hole across a type II band-aligned semiconductor heterostructure leads to an unexpected sub-200-femtosecond upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this upshift is a clear fingerprint of the correlated interaction of the electron and hole parts of the exciton. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access electron-hole correlations and cooperative behavior in quantum materials. Our work highlights this capability and motivates the future study of optically inaccessible correlated excitonic and electronic states of matter.

Exponential relaxation of the energy and desorption dynamics of atoms colliding with a surface

Atom–surface collisions are one of the most important topics in surface science. To further disclose the physical mechanism underlying atom–surface interaction at the microscopic level, we study the dynamics of an incident atom with a molecular dynamics simulation. Emphasis is put on the temporal evolutions of energy and residence times of the colliding atoms. The incident atoms experience two stages after colliding with the surface. First, the atoms relax to the equilibrium state in an exponential fashion. Then, the atoms become equilibrated with the surface and depart from the surface with a converged desorption rate. Two parameters are proposed to characterize the process: the characteristic energy relaxation time and the equilibrium residence time. At the relaxation stage, the desorption rate varies with the energy, and the probability distribution function (PDF) of the residence time obeys a power law. At the equilibrium state, the desorption rate is invariable, and the PDF of the residence time decays exponentially. We further find that the desorption rate for both stages can be calculated by a consistent Arrhenius equation, with the desorption activation energy and kinetic energy evolving with time in the relaxation stage. It appears that the gas–surface interaction dynamics can be explained by trapping-desorption theory in both the relaxation state and the equilibration state.

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene

Understanding electron-phonon interactions is fundamentally important and has crucial implications for device applications. However, in twisted bilayer graphene near the magic angle, this understanding is currently lacking. Here, we study electron-phonon coupling using time- and frequency-resolved photovoltage measurements as direct and complementary probes of phonon-mediated hot-electron cooling. We find a remarkable speedup in cooling of twisted bilayer graphene near the magic angle: The cooling time is a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. Our experimental and theoretical analysis indicates that this ultrafast cooling is a combined effect of superlattice formation with low-energy moiré phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone. This enables efficient electron-phonon Umklapp scattering that overcomes electron-phonon momentum mismatch. These results establish twist angle as an effective way to control energy relaxation and electronic heat flow.