Modulating Thermal Conductivity via Targeted Phonon Excitation

Thermal conductivity is a critical material property in numerous applications, such as those related to thermoelectric devices and heat dissipation. Effectively modulating thermal conductivity has become a great concern in the field of heat conduction. Here, a quantum modulation strategy is proposed to modulate the thermal conductivity/heat flux by exciting targeted phonons. It shows that the thermal conductivity of graphene can be tailored in the range of 1559 W m-1 K-1 (decreased to 49%) to 4093 W m-1 K-1 (increased to 128%), compared with the intrinsic value of 3189 W m-1 K-1. The effects are also observed for graphene nanoribbons and bulk silicon. The results are obtained through both density functional theory calculations and molecular dynamics simulations. This novel modulation strategy may pave the way for quantum heat conduction.

Plasmon Squeezing in Single-Molecule Junctions

Scanning tunneling microscope (STM)-induced luminescence provides an ideal platform for electrical generation and the atomic-scale manipulation of nonclassical states of light. However, despite its extreme importance in quantum technologies, squeezed light emission with reduced quantum fluctuations has hitherto not been demonstrated in such a platform. Here, we theoretically predict that the emitted light from the plasmon mode can be squeezed in an STM single molecular junction subject to an external laser drive. Going beyond the traditional paradigm that generates squeezing with the quadratic interaction of photons, our prediction explores the molecular coherence involved in an anharmonic energy spectrum of a coupled plasmon-molecule-exciton system. Furthermore, we show that, by selectively exciting the energy ladder, the squeezed plasmon can show either sub- or super-Poissonian statistical properties. We also demonstrate that, following the same principle, the molecular excitonic mode can be squeezed simultaneously.

Order of magnitude reduction in Joule heating of single molecular junctions between graphene electrodes

Maintaining stability of single-molecular junctions (SMJs) in the presence of current flow is a prerequisite for their potential device applications. But theoretical understanding of nonequilibrium heat transport in current-carrying SMJs is a challenging problem due to different kinds of nonlinear interaction involved, including electron-vibration and anharmonic vibrational coupling.Here, we overcome this challenge by accelerating Langevin-type current-induced molecular dynamics using machine-learning potential derived from density functional theory. We show that SMJs with graphene electrodes generate order of magnitude less heating than those with gold electrodes. This is rooted in better phonon spectral overlap of graphene with molecular vibrations, rendering harmonic phonon heat transport being dominant. In contrast, in spectrally mismatched junction with gold electrodes, anharmonic coupling becomes important to transport heat away from the molecule to surrounding electrodes. Our work paves the way of studying current-induced heat transport and energy redistribution in realistic SMJs.

Observation of Biradical Spin Coupling through Hydrogen Bonds

Investigation of intermolecular electron spin interaction is of fundamental importance in both science and technology. Here, radical pairs of all-trans retinoic acid molecules on Au(111) are created using an ultralow temperature scanning tunneling microscope. Antiferromagnetic coupling between two radicals is identified by magnetic-field-dependent spectroscopy. The measured exchange energies are from 0.1 to 1.0 meV. The biradical spin coupling is mediated through O─H⋯O hydrogen bonds, as elucidated from analysis combining density functional theory calculation and a modern version of valence bond theory.

Charge Transfer Gap Tuning via Structural Distortion in Monolayer 1T-NbSe2

The Mott state in 1T-TaS₂ is predicted to host quantum spin liquids (QSLs). However, its insulating mechanism is controversial due to complications from interlayer coupling. Here, we study the charge transfer state in monolayer 1T-NbSe₂, an electronic analogue to TaS₂ exempt from interlayer coupling, using spectroscopic imaging scanning tunneling microscopy and first-principles calculations. Monolayer NbSe₂ surprisingly displays two types of star of David (SD) motifs with different charge transfer gap sizes, which are interconvertible via temperature variation. In addition, bilayer 1T-NbSe₂ shows a Mott collapse by interlayer coupling. Our calculation unveils that the two types of SDs possess distinct structural distortions, altering the effective Coulomb energies of the central Nb orbital. Our calculation suggests that the charge transfer gap, the same parameter for determining the QSL regime, is tunable with strain. This finding offers a general strategy for manipulating the charge transfer state in related systems, which may be tuned into the potential QSL regime.

Effective Control of Photon Statistics from Electroluminescence by Fano-like Interference Effect

The photon blockade induced by optical nonlinearity has been widely used to generate single-photon emission under optical driving in quantum optics. However, the same approach is difficult to achieve in electrically driven molecular junctions. Here we propose a scheme for tuning photon statistics via Fano-like interference effect in a system consisting of two molecules within one optical cavity. Under electrical pumping, a transition from photon bunching to antibunching takes place as a manifestation of the Fano-like interference. This effect persists even in the presence of the dipole-dipole interaction between molecules based on the parameters extracted from the experiments. Our proposal can be realized in current-carrying scanning tunneling microscope junctions.

Nonthermal vibrations in biased molecular junctions

We study vibrational statistics in current-carrying model molecular junctions using a master equation approach. In particular, we concentrate on the validity of using an effective temperature T_{eff} to characterize the nonequilibrium steady state of a vibrational mode. We identify cases in which a single T_{eff} cannot fully describe one vibrational state. In such cases, the probability distribution among different vibrational states does not follow the Boltzmann type. Consequently, the actual entropy (free energy) of the vibrational mode is lower (higher) than the corresponding thermal value given by T_{eff}, indicating extra work can be extracted from these states. Our results will be useful for the study of a nonthermal vibrational state in the thermodynamics of nanoscale systems, and its usage in nanoscale heat engines.

Heat vortex in hydrodynamic phonon transport of two-dimensional materials

We study hydrodynamic phonon heat transport in two-dimensional (2D) materials. Starting from the Peierls-Boltzmann equation with the Callaway model approximation, we derive a 2D Guyer-Krumhansl-like equation describing hydrodynamic phonon transport, taking into account the quadratic dispersion of flexural phonons. In addition to Poiseuille flow, second sound propagation, the equation predicts heat current vortices and negative non-local thermal conductance in 2D materials, which are common in classical fluids but have not yet been considered in phonon transport. Our results also illustrate the universal transport behaviors of hydrodynamics, independent of the type of quasi-particles and their microscopic interactions.

Spin-Dependent Transport in van der Waals Magnetic Tunnel Junctions with Fe3GeTe2 Electrodes

van der Waals (vdW) heterostructures, stacking different two-dimensional materials, have opened up unprecedented opportunities to explore new physics and device concepts. Especially interesting are recently discovered two-dimensional magnetic vdW materials, providing new paradigms for spintronic applications. Here, using density functional theory (DFT) calculations, we investigate the spin-dependent electronic transport across vdW magnetic tunnel junctions (MTJs) composed of Fe₃GeTe₂ ferromagnetic electrodes and a graphene or hexagonal boron nitride (h-BN) spacer layer. For both types of junctions, we find that the junction resistance changes by thousands of percent when the magnetization of the electrodes is switched from parallel to antiparallel. Such a giant tunneling magnetoresistance (TMR) effect is driven by dissimilar electronic structure of the two spin-conducting channels in Fe₃GeTe₂, resulting in a mismatch between the incoming and outgoing Bloch states in the electrodes and thus suppressed transmission for an antiparallel-aligned MTJ. The vdW bonding between electrodes and a spacer layer makes this result virtually independent of the type of the spacer layer, making the predicted giant TMR effect robust with respect to strain, interface distance, and other parameters, which may vary in the experiment. We hope that our results will further stimulate experimental studies of vdW MTJs and pave the way for their applications in spintronics.

Intrinsically low thermal conductivity of bismuth oxychalcogenides originating from interlayer coupling

Strong interlayer anharmonic coupling leads to intrinsically low thermal conductivity of bismuth oxychalcogenides.

On the Fano Line Shape of Single Molecule Electroluminescence Induced by a Scanning Tunneling Microscope

The coupling between molecular exciton and gap plasmons plays a key role in single molecular electroluminescence induced by a scanning tunneling microscope (STM). But it has been difficult to clarify the complex experimental phenomena. By employing the nonequilibrium Green's function method, we propose a general theoretical model to understand the light emission spectrum of single molecule and gap plasmons from an energy transport point of view. The coherent interaction between gap plasmons and molecular exciton leads to a prominent Fano resonance in the emission spectrum. We analyze the dependence of the Fano line shape on the system parameters, based on which we provide a unified account of several recent experimental observations. Moreover, we highlight the effect of the tip-molecule electronic coupling on the spectrum.

Decay channels of gap plasmons in STM tunnel junctions

We study the decay of gap plasmons localized between a scanning tunneling microscope tip and metal substrate, excited by inelastic tunneling electrons. The overall excited energy from the tunneling electrons is divided into two categories in the form of resistive dissipation and electromagnetic radiation, which together can further be separated into four diffierent channels, including SPP channel on the tip, SPP channel on the substrate, air mode channel and direct quenching channel. In this work, we study the enhancement factor, i.e. Purcell factor, of the STM tunnel junctions, which are mediated by the nearby metallic structures. We find that the gap plasmon mode is most likely to couple to the SPP channel on the tip, rather than the SPP channel on the substrate or the air mode. The direct quenching in the apex of tip also takes a considerable portion especially in high frequency region, the enhancement factor of direct quenching in the tip is much higher than the direct quenching in the substrate. We adopt four tips with diffierent apex radii, i.e., 1 nm, 5 nm, 10 nm, 20 nm. When the apex size is small, the frequency dependent enhancement factor from the SPPs contribution has a pronounced peak at 1.55 eV, however, as the radius increases, the peak of enhancement factor in the high frequency region appears, the 1.55 eV peak becomes less dominated. This phenomenon can be attributed to the change of tip shape, in the form of mode coupling. Our results also show a relationship between the direct quenching in the substrate and in the tip. With the larger radius of apex, the ratio of these two part of energy approaches 1, which indicate that the energy distribution of direct quenching is sensitive to the shape of the tip-substrate gap.

Optical Phonon Behaviors of Photocharged Nanocrystals: Effects of Free Charge Carriers

For semiconductor nanocrystals (NCs), the precise knowledge of phonons in the presence of free carriers is important for understanding their electronic and photonic properties in device applications. With Raman spectroscopy, this study investigates the effects of free charge carriers on optical phonon behaviors of NCs. The adoption of the photocharging method allows us to introduce free charge carriers into NCs without inducing other side effects. In the photocharged ZnO NCs, lower longitudinal optical (LO) phonon frequencies and weaker LO overtones relative to the fundamentals were found, which was explained by the screening and band-filling effects caused by the induced free carriers. The free carrier effects on optical phonon behaviors of NCs, usually neglected in previous studies, should be taken into consideration when discussing the electronic and photonic properties of NC-based devices.

Coulomb-force-mediated heat transfer in the near field: Geometric effect

It has been shown recently that the Coulomb part of electromagnetic interactions is more important than the transverse propagation waves for the near-field enhancement of heat transfer between metal objects at a distance of order nanometers. Here we present a theory focusing solely on the Coulomb potential between electrons hopping among tight-binding sites. When the relevant systems are reduced to very small geometry, for example, a single site, the enhancement is much higher compared to a collection of them packed within a distance of a few Å. We credit this to the screening effect. This result may be useful in designing metal-based metamaterials to enhance heat transfer much higher.

Strain-induced thermoelectric performance enhancement of monolayer ZrSe2

Enhancing the thermoelectric performance of monolayer ZrSe₂via strain induced band valley engineering.

Vacuum synthesis of magnetic aluminum phthalocyanine on Au(111)

Air-unstable magnetic aluminum phthalocyanine (AlPc) molecules are prepared by an on-surface metalation reaction of phthalocyanine with aluminum (Al) atoms on Au(111) in ultrahigh vacuum. Experiments and density functional theory calculations show that an unpaired spin is located on the conjugated isoindole lobes of the molecule rather than at the Al position.

Nuclear quantum effects of hydrogen bonds probed by tip-enhanced inelastic electron tunneling

We report the quantitative assessment of nuclear quantum effects on the strength of a single hydrogen bond formed at a water-salt interface, using tip-enhanced inelastic electron tunneling spectroscopy based on a scanning tunneling microscope. The inelastic scattering cross section was resonantly enhanced by "gating" the frontier orbitals of water via a chlorine-terminated tip, so the hydrogen-bonding strength can be determined with high accuracy from the red shift in the oxygen-hydrogen stretching frequency of water. Isotopic substitution experiments combined with quantum simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the relatively strong ones. However, this trend can be completely reversed when a hydrogen bond is strongly coupled to the polar atomic sites of the surface.

Current-induced runaway vibrations in dehydrogenated graphene nanoribbons

We employ a semi-classical Langevin approach to study current-induced atomic dynamics in a partially dehydrogenated armchair graphene nanoribbon. All parameters are obtained from density functional theory. The dehydrogenated carbon dimers behave as effective impurities, whose motion decouples from the rest of carbon atoms. The electrical current can couple the dimer motion in a coherent fashion. The coupling, which is mediated by nonconservative and pseudo-magnetic current-induced forces, change the atomic dynamics, and thereby show their signature in this simple system. We study the atomic dynamics and current-induced vibrational instabilities using a simplified eigen-mode analysis. Our study illustrates how armchair nanoribbons can serve as a possible testbed for probing the current-induced forces.

Spin Manipulation by Creation of Single-Molecule Radical Cations

All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron.

Controlling Molecular Growth between Fractals and Crystals on Surfaces

Recent studies demonstrate that simple functional molecules, which usually form two-dimensional (2D) crystal structures when adsorbed on solid substrates, are also able to self-assemble into ordered openwork fractal aggregates. To direct and control the growth of such fractal supramolecules, it is necessary to explore the conditions under which both fractal and crystalline patterns develop and coexist. In this contribution, we study the coexistence of Sierpiński triangle (ST) fractals and 2D molecular crystals that were formed by 4,4″-dihydroxy-1,1':3',1″-terphenyl molecules on Au(111) in ultrahigh vacuum. Growth competition between the STs and 2D crystals was realized by tuning substrate and molecular surface coverage and changing the functional groups of the molecular building block. Density functional theory calculations and Monte Carlo simulations are used to characterize the process. Both experimental and theoretical results demonstrate the possibility of steering the surface self-assembly to generate fractal and nonfractal structures made up of the same molecular building block.

Enhancing the Thermoelectric Figure of Merit by Low-Dimensional Electrical Transport in Phonon-Glass Crystals

Low-dimensional electronic and glassy phononic transport are two important ingredients of highly efficient thermoelectric materials, from which two branches of thermoelectric research have emerged. One focuses on controlling electronic transport in the low dimension, while the other focuses on multiscale phonon engineering in the bulk. Recent work has benefited much from combining these two approaches, e.g., phonon engineering in low-dimensional materials. Here we propose to employ the low-dimensional electronic structure in bulk phonon-glass crystals as an alternative way to increase the thermoelectric efficiency. Through first-principles electronic structure calculations and classical molecular dynamics simulations, we show that the π-π-stacking bis(dithienothiophene) molecular crystal is a natural candidate for such an approach. This is determined by the nature of its chemical bonding. Without any optimization of the material parameters, we obtained a maximum room-temperature figure of merit, ZT, of 1.48 at optimal doping, thus validating our idea.

The integrated spintronic functionalities of an individual high-spin state spin-crossover molecule between graphene nanoribbon electrodes

The spin-polarized transport properties of a high-spin-state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes have been studied using density functional theory combined with the nonequilibrium Green's-function formalism. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and the spin rectifying effect, associated with the giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost unaffected by the electrode temperature. The microscopic mechanism of these functionalities is discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of the individual spin-crossover molecule.

Current-induced forces and hot spots in biased nanojunctions

We investigate theoretically the interplay of current-induced forces (CIFs), Joule heating, and heat transport inside a current-carrying nanoconductor. We find that the CIFs, due to the electron-phonon coherence, can control the spatial heat dissipation in the conductor. This yields a significant asymmetric concentration of excess heating (hot spot) even for a symmetric conductor. When coupled to the electrode phonons, CIFs drive different phonon heat flux into the two electrodes. First-principles calculations on realistic biased nanojunctions illustrate the importance of the effect.

Spectroscopy of transmission resonances through a C₆₀ junction

Electron transport through a single C60 molecule on Cu(1 1 1) has been investigated with a scanning tunnelling microscope in tunnelling and contact ranges. Single-C60 junctions have been fabricated by establishing a contact between the molecule and the tip, which is reflected by a down-shift in the lowest unoccupied molecular orbital resonance. These junctions are stable even at elevated bias voltages enabling conductance measurements at high voltages and nonlinear conductance spectroscopy in tunnelling and contact ranges. Spectroscopy and first principles transport calculations clarify the relation between molecular orbital resonances and the junction conductance. Due to the strong molecule-electrode coupling the simple picture of electron transport through individual orbitals does not hold.

Thermal transport across metal–insulator interface via electron–phonon interaction

The thermal transport across a metal–insulator interface can be characterized by electron–phonon interaction through which an electron lead is coupled to a phonon lead if phonon–phonon coupling at the interface is very weak. We investigate the thermal conductance and rectification between the electron part and the phonon part using the nonequilibrium Green's function method. It is found that the thermal conductance has a nonmonotonic behavior as a function of average temperature or the coupling strength between the phonon leads in the metal part and the insulator part. The metal–insulator interface shows a clear thermal rectification effect, which can be reversed by a change in average temperature or the electron–phonon coupling.

Light emission probing quantum shot noise and charge fluctuations at a biased molecular junction

The emission of plasmonic light from a single C(60) molecule on Cu(111) is probed in a scanning tunneling microscope from the weak-coupling, tunneling range to strong coupling of the molecule to the electrodes at contact. At positive sample voltage the photon yield decreases owing to shot-noise suppression in an increasingly transparent quantum contact. At reversed bias an unexpected nonlinear increase occurs. First-principles transport calculations reveal that ultrafast charge fluctuations on the molecule give rise to additional noise at optical frequencies beyond the shot noise of the current that is injected to the tip.

Current-induced dynamics in carbon atomic contacts

BACKGROUND

The effect of electric current on the motion of atoms still poses many questions, and several mechanisms are at play. Recently there has been focus on the importance of the current-induced nonconservative forces (NC) and Berry-phase derived forces (BP) with respect to the stability of molecular-scale contacts. Systems based on molecules bridging electrically gated graphene electrodes may offer an interesting test-bed for these effects.

RESULTS

We employ a semi-classical Langevin approach in combination with DFT calculations to study the current-induced vibrational dynamics of an atomic carbon chain connecting electrically gated graphene electrodes. This illustrates how the device stability can be predicted solely from the modes obtained from the Langevin equation, including the current-induced forces. We point out that the gate offers control of the current, independent of the bias voltage, which can be used to explore current-induced vibrational instabilities due the NC/BP forces. Furthermore, using tight-binding and the Brenner potential we illustrate how Langevin-type molecular-dynamics calculations including the Joule heating effect for the carbon-chain systems can be performed. Molecular dynamics including current-induced forces enables an energy redistribution mechanism among the modes, mediated by anharmonic interactions, which is found to be vital in the description of the electrical heating.

CONCLUSION

We have developed a semiclassical Langevin equation approach that can be used to explore current-induced dynamics and instabilities. We find instabilities at experimentally relevant bias and gate voltages for the carbon-chain system.

Laserlike vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.

Blowing the fuse: Berry's phase and runaway vibrations in molecular conductors

We examine a molecular bridge connecting two metallic electrodes. We find that an electronic current passing across the bridge can cause a vibrational instability of the molecule, which ultimately can lead to a breakdown of the bridge. This instability is generated by a hitherto never considered mechanism, which surprisingly involves the quantum mechanical phase of the electronic waves, the "Berry phase". This mechanism works for highly conducting bridges, and contrary to breakdown by traditional Joule heating, this instability is deterministic and occurs at certain critical voltages. We demonstrate the new mechanism using state-of-the-art ab initio calculations on realistic molecular bridges.

Coupled electron-phonon transport from molecular dynamics with quantum baths

Based on generalized quantum Langevin equations for the tight-binding wavefunction amplitudes and lattice displacements, electron and phonon quantum transport are obtained exactly using molecular dynamics (MD) in the ballistic regime. The electron-phonon interactions can be handled with a quasi-classical approximation. Both charge and energy transport and their interplay can be studied. We compare the MD results with those of a fully quantum mechanical nonequilibrium Green's function (NEGF) approach for the electron currents. We find a ballistic to diffusive transition of the electron conduction in one-dimensional chains as the chain length increases.

[Bacteriological analysis of chronic sinusitis in school-age children (36 cases report)]

OBJECTIVE

To research the bacteriological characters of chronic sinusitis in school-age children and to provide basis for clinical treatment.

METHOD

The purrent discharges taken from middle meatus of 36 patients were cultured for bacteria and the antimicrobial suscepility was determined.

RESULT

The positive rate of bacteria culture was 83.3%, the compound infection rate was 36.1%. Aerobic bacteria were present in 76.7%, anaerobic bacteria were present in 55.6%. The result of antimicrobial susceptibility test was scattered.

CONCLUSION

Infection rate of anaerobic bacteria was high in chronic sinusitis in school-age children and result of antimicrobial susceptibility was scattered. It was important to have anti-anaerobic treatment in chronic sinusitis in school-age children and to take an antimicrobial susceptibility test before medical treatment.