Electronic structure and phase transition engineering in NbS2: Crucial role of van der Waals interactions

Based on first-principles simulations,we revisit the crystal structures,electronic structures,and structural stability of the layered transition metal dichalcogenides(TMDCs)NbS2,and shed more light on the crucial roles of the van der Waals(vdW)interactions.Theoretically calculated results imply that the vdW corrections are important to reproduce the layered crystal structure,which is significant to correctly describe the electronic structure of NbS2.More interestingly,under hydrostatic pressure or tensile strain in ab plane,an isostructural phase transition from two-dimensional layered structure to three-dimensional bulk in the I4/mmm phase has been uncovered.The abnormal structural transition is closely related to the electronic structure instability and interlayer bonding effects.The interlayer Nb-S distances collapse and the interlayer vdW interactions disappear,concomitant with new covalent bond emerging and increasing coordination number.Present work highlights the significance of the vdW interactions,and provides new insights on the unconventional structural transitions in NbS2,which will attract wide audience working in the hectic field of TMDCs.

Phonon resonance modulation in weak van der Waals heterostructures:Controlling thermal transport in graphene-silicon nanoparticle systems

The drive for efficient thermal management has intensified with the miniaturization of electronic devices.This study explores the modulation of phonon transport within graphene by introducing silicon nanoparticles influenced by van der Waals forces.Our approach involves the application of non-equilibrium molecular dynamics to assess thermal conductivity while varying the interaction strength,leading to a noteworthy reduction in thermal conductivity.Furthermore,we observe a distinct attenuation in length-dependent behavior within the graphene-nanoparticles system.Our exploration combines wave packet simulations with phonon transmission calculations,aligning with a comprehensive analysis of the phonon transport regime to unveil the underlying physical mechanisms at play.Lastly,we conduct transient molecular dynamics simulations to investigate interfacial thermal conductance between the nanoparticles and the graphene,revealing an enhanced thermal boundary conductance.This research not only contributes to our understanding of phonon transport but also opens a new degree of freedom for utilizing van der Waals nanoparticle-induced resonance,offering promising avenues for the modulation of thermal properties in advanced materials and enhancing their performance in various technological applications.

Superradiance of ultracold cesium Rydberg |65D_(5/2)> → |66P_(3/2)>

We investigate Rydberg |65D_(5/2)> → |66P_(3/2)> superradiance in dense ultracold cesium atoms,where the ground atoms are excited to |65D_(5/2)> Rydberg states via two-photon excitation in a standard magneto-optical trap.The superradiant spectrum of |65D_(5/2)> → |66P_(3/2)> is obtained using the state-selective field ionization technique.We observe its dynamic evolution process by varying the delay time of ionization field td.The results show that the evolution process of |65D_(5/2)> →|66P_(3/2)> is much shorter than its radiation lifetime at room temperature,which verifies the superradiance effect.The dependence of the superradiance process on Rydberg atoms number N_(e) and principal quantum number n is investigated.The results show that the superradiance becomes faster with increasing N_(e),while it is suppressed for stronger van der Waals(vdW) interactions.Superradiance has potential applications in quantum technologies,and the Rydberg atom is an ideal medium for superradiance.Our system is effective for studying the strong two-body interaction between Rydberg atoms.

Alloy-buffer-controlled van der Waals epitaxial growth of aligned tellurene

Group-VI elemental two-dimensional(2D)materials(e.g.,tellurene(Te))have unique crystalline structures and extraordinarily physical properties.However,it still remains a great challenge to controllably grow 2D Te with good repeatability,uniformity,and highly aligned orientation using vapor growth method.Here,we design a Cu foil-assisted alloy-buffer-controlled growth method to epitaxially grow aligned single-crystalline 2D Te on an insulating mica substrate.The in-situ formation of Cu-Te alloy plays a key role on 2D Te growth,alleviating the spatial and temporal non-uniformity of precursor in conventional vapor deposition process.Through transmission electron microscopy(TEM)analysis combined with theoretical calculations,we unveil that the alignment growth of Te in the[110]direction is along the[600]direction of mica,owing to the small lattice mismatch(0.15%)and strong binding strength.This work presents a method to grow aligned high-quality 2D Te in a controllable manner.

Theoretical calculations of thermophysical properties of single-wall carbon nanotube bundles

Carbon nanotube bundles are promising thermal interfacial materials due to their excellent thermal and mechanical characteristics. In this study, the phonon dispersion relations and density of states of the single-wall carbon nanotube bundles are calculated by using the force constant model. The calculation results show that the inter-tube interaction leads to a significant frequency raise of the low frequency modes. To verify the applied calculation method, the specific heat of a single single-wall carbon nanotube is calculated first based on the obtained phonon dispersion relations and the results coincide well with the experimental data. Moreover, the specific heat of the bundles is calculated and exhibits a slight reduction at low temperatures in comparison with that of the single tube. The thermal conductivity of the bundles at low temperatures is calculated by using the ballistic transport model. The calculation results indicate that the inter-tube interaction, i.e. van der Waals interaction, hinders heat transfer and cannot be neglected at extremely low temperatures. For （5, 5） bundles, the relative difference of the thermal conductivity caused by ignoring inter-tube effect reaches the maximum value of 26% around 17 K, which indicates the significant inter-tube interaction effect on the thermal conductivity at low temperatures.

Quantum thermal field fluctuation induced corrections to the interaction between two ground-state atoms

We generalize the formalism proposed by Dalibard, Dupont-Roc, and Cohen-Tannoudji(the DDC formalism) in the fourth order for two atoms in interaction with scalar fields in vacuum to a thermal bath at finite temperature T, and then calculate the interatomic interaction energy of two ground-state atoms separately in terms of the contributions of thermal fluctuations and the radiation reaction of the atoms and analyze in detail the thermal corrections to the van der Waals and Casimir–Polder interactions. We discover a particular region, i.e. 4(λ3β)(1/2) ■ L■λwith L, β and λ denoting the interatomic separation, the wavelength of thermal photons and the transition wavelength of the atoms respectively, where the thermal corrections remarkably render the van der Waals force, which is usually attractive, repulsive, leading to an interesting crossover phenomenon of the interatomic interaction from attractive to repulsive as the temperature increases. We also find that the thermal corrections cause significant changes to the Casimir–Polder force when the temperature is sufficiently high, resulting in an attractive force proportional to TL-3in the λ ■ β ■ L region, and a force that can be either attractive or repulsive and even vanishing in the β ■ λ ■ L region depending on the interatomic separation.

Transformation optics from macroscopic to nanoscale regimes:a review

Transformation optics is a mathematical method that is based on the geometric interpretation of Maxwell’s equations.This technique enables a direct link between a desired electromagnetic(EM)phenomenon and the material response required for its occurrence,providing a powerful and intuitive design tool for the control of EM fields on all length scales.With the unprecedented design flexibility offered by transformation optics(TO),researchers have demonstrated a host of interesting devices,such as invisibility cloaks,field concentrators,and optical illusion devices.Recently,the applications of TO have been extended to the subwavelength scale to study surface plasmon-assisted phenomena,where a general strategy has been suggested to design and study analytically various plasmonic devices and investigate the associated phenomena,such as nonlocal effects,Casimir interactions,and compact dimensions.We review the basic concept of TO and its advances from macroscopic to the nanoscale regimes.

Thermal Transport across Polyethylene Chains

In polymers,heat could transfer efficiently along the long polymer chains;however due to the finite length of polymer chains,such heat eventually has to pass across the chain-chain boundary which is less effective in heat transfer.This paper investigated the thermal transport across polyethylene chains with molecular dynamics(MD)simulations.Thermal transport across two polymer chains overlapping with each other is studied with different chain length(75 nm,150 nm and 251 nm)and chain-chain overlapping length.The results show that with increasing overlapping length,the total thermal conductance across the two chains exhibits maximum value,which is due to the increasing thermal resistance along the chains and the decreasing inter-chain thermal boundary resistance.Mathematically,we show that the total thermal resistance can be decomposed into two terms.The coupling term related to the inter-chain thermal resistance tends to saturate even with long overlapping length.