Search Results (69 total)

We predict a magnetoelectric (ME) effect in graphene nanoribbons on silicon substrates by first-principles calculations. It is shown that a bias voltage can produce strong linear ME effect by driving charge transfer between the nanoribbons and substrate, thus tuning the exchange splitting of magnetic edge states; moreover, the bias induced n-to-p-type transition in the ribbon layer can switch the ME coefficient from negative to positive due to the unique symmetry of band structures. This mechanism is proven to be robust against variations in material and physical configurations, thus opening a new avenue for ME coupling in metal-free magnet systems of practical importance.

It is well known that, under the rotating-wave approximation, the Rabi Hamiltonian, which describes the interaction between a two-level atom and a single field mode, reduces to the Jaynes-Cummings model through exclusion of its counter-rotating term. In this work, it is found that, since photons in the mode are indistinguishable, there exist two conditions where the counter-rotating term can never be ignored. These conditions, all subject to the requirements that the atom-mode coupling is weak and the atom is initially in its ground state, are obtained in the stationary perturbation theory and confirmed through an analysis of the atom-mode system’s evolution.

We show by extensive first-principles calculations that an n- to p-type transition in epitaxial bilayer graphene can be induced by applying bias voltage on C-terminated SiC substrate, but cannot occur on Si-terminated SiC. Bias voltage can cause enough charge transfer between top and bottom graphene layers on C-terminated SiC to shift the Dirac level below or above the Fermi level. On both C- and Si-terminated SiC, change in interlayer spacing of the epitaxial bilayer graphene produces charge redistribution that leads to large increase in the energy gap, but cannot raise the Dirac level efficiently. The surface terminated condition or properties of substrate are of essential importance in possible gate tuning electronic behavior of epitaxial graphene on it.

A Reply to the Comment by Feng Ding, Jianyu Huang, and Boris I. Yakobson.

We study a percolation problem based on critical loop configurations of the O(n) loop model on the honeycomb lattice. We define dual clusters as groups of sites on the dual triangular lattice that are not separated by a loop, and investigate the bond-percolation properties of these dual clusters. The universal properties at the percolation threshold are argued to match those of Kasteleyn-Fortuin random clusters in the critical Potts model. This relation is checked numerically by means of cluster simulations of several O(n) models in the range 1≤n≤2. The simulation results include the percolation threshold for several values of n, as well as the universal exponents associated with bond dilution and the size distribution of the diluted clusters at the percolation threshold. Our numerical results for the exponents are in agreement with existing Coulomb-gas results for the random-cluster model, which confirms the relation between both models. We discuss the renormalization flow of the bond-dilution parameter p as a function of n, and provide an expression that accurately describes a line of unstable fixed points as a function of n, corresponding with the percolation threshold. Furthermore, the renormalization scenario indicates the existence, in a p versus n diagram, of another line of fixed points at p=1, which is stable with respect to p.

We derive the scaling dimension associated with crossing bonds in the random-cluster representation of the two-dimensional Potts model by means of a mapping on the Coulomb gas. The scaling field associated with crossing bonds appears to be irrelevant on the critical as well as on the tricritical branch. The latter result stands in a remarkable contrast with the existing result for the tricritical O(n) model that crossing bonds are relevant. Although the O(1) model is equivalent with the q=2 random-cluster model, the crossing-bond exponents obtained for these two models appear to be different. We provide an explanation of this peculiar observation. In order to obtain an independent confirmation of the Coulomb gas result for the crossing-bond exponent, we perform a finite-size-scaling analysis based on numerical transfer-matrix calculations.

Metastable helium molecules generated in a discharge near a sharp tungsten tip immersed in superfluid ⁴He are imaged using a laser-induced-fluorescence technique. By pulsing the tip, a small cloud of He₂^* molecules is produced. We can determine the normal-fluid velocity in a heat-induced counterflow by tracing the position of a single molecule cloud. As we run the tip in continuous field-emission mode, a normal-fluid jet from the tip is generated and molecules are entrained in the jet. A focused 910 nm pump laser pulse is used to drive a small group of molecules to the first excited vibrational level of the triplet ground state. Subsequent imaging of the tagged molecules with an expanded 925 nm probe laser pulse allows us to measure the flow velocity of the jet. The techniques we developed provide new tools in quantitatively studying the normal fluid flow in superfluid helium.

We present a study of smectic focal conic domains in films of various liquid-crystal compounds on a solid substrate. Focal conic domains are generated as a result of the antagonistic anchoring conditions of the film surfaces, random planar at the substrate interface and homeotropic at the air interface. The curved arrangement of the smectic layers in focal conic domains leads to a depression in the film/air interface above each domain. Using atomic force microscopy, we determine the temperature dependence of the depth of the depressions in different smectic phases and at different phase transitions. In most cases, our results are not in accordance with the assumption of strictly incompressible smectic layers, and we discuss the observed behavior with respect to the known results on the layer compressibility modulus B.

We report molecular dynamics simulations of tensile elongation of carbon nanotubes (CNTs) over a wide temperature range. In particular, we examine temperature and size effects on tensile ductility of CNTs and compare our results with recent experimental observation on superplastic deformation of CNTs at high temperatures. Our simulations produce substantial tensile ductility in CNTs with large diameters at high temperatures and reveal that similar behavior can be realized over a surprisingly large temperature range between 500 and 2400 K that is yet to be fully explored by experiments. At lower temperatures, tensile deformation modes become brittle due to defect localization attributed to insufficient thermal energy for wide distribution of defect nucleation. For CNTs with smaller diameters, our simulations produce strong defect localization which leads to brittle behavior even at high temperatures. Sensitive dependence on the distribution of incipient defects on thermal energy results in a significant decrease in the elastic limit with increasing temperature. We propose an effective tensile ductility enhancement via temperature reduction beyond the elastic limit. The results offer insights for understanding intriguing temperature effects on tensile deformation modes of CNTs.

High-spin level structures of ^94,95Mo have been reinvestigated via the ¹⁶O(⁸²Se,xnγ)^94,95Mo(x=4,3) reactions at E(⁸²Se)=460 MeV. The previously reported level schemes of these two nuclei have been largely modified up to ∼11 MeV in excitation energy due to identifications of some important linking transitions. Shell-model calculations have been made in the model space of π(p_1/2,g_9/2,d_5/2)⁴ and ν(d_5/2,s_1/2,d_3/2,g_7/2,h_11/2)²⁽³⁾ and compared with the modified level schemes. The structures of the newly assigned high-spin states in ^94,95Mo have been discussed.

We describe experiments we have performed in which we are able to image the motion of individual electrons moving in liquid helium 4. Electrons in helium form bubbles of radius ~19 Å. We use the negative pressure produced by a sound wave to expand these bubbles to a radius of about 10 μm. The bubbles are then illuminated with light from a flash lamp and their position recorded. We report on several interesting phenomena that have been observed in these experiments. It appears that the majority of the electrons that we detect result from cosmic rays passing through the experimental cell. We discuss this mechanism for electron production.

Phase stabilities and mechanical properties of ideal stoichiometric technetium monocarbide (TcC) and technetium mononitride (TcN) in the tungsten carbide (WC), nickel arsenide (NiAs), rocksalt (NaCl), and zinc-blende (ZnS) structures, respectively, have been studied systematically by first-principles calculations. It is found that both TcC and TcN in two hexagonal phases (WC and NiAs) are not only elastically stable but also hard and ultrastiff materials. Remarkably, for the two hexagonal TcC phases, both bulk moduli and linear incompressibilities along the c axis exceed that of c BN and even rival with diamond. Their hardness can also match the known hard materials such as WC. The combination of good metallicity, strong stiffness, and high hardness suggests that the materials may find applications as hard conductors and cutting tools.

We present a study of focal conic domains in smectic-A liquid-crystal films on solid substrates. The antagonistic anchoring conditions of the film surfaces, random planar at the substrate interface and homeotropic at the air interface, enforce the formation of focal conic domains the lateral size of which is dependent on the film thickness. The strength of the planar anchoring on the solid substrate is systematically varied by coating the substrate with special alkoxysilane compounds. For each anchoring strength value, the relation between the size of the focal conic domains and the film thickness is determined. Increasing the planar anchoring strength influences the size-thickness relation and leads to the formation of larger focal conic domains.

We show that the exactly solved low-temperature branch of the two-dimensional O(n) model is equivalent to an O(n) model with vacancies and a different value of n. We present analytic results for several universal parameters of the latter model, which is identified as a tricritical point. These results apply to the range n≤3∕2 and include the exact tricritical point, the conformal anomaly, and a number of scaling dimensions, among which are the thermal and magnetic exponents, and the exponent associated with the crossover to ordinary critical behavior and to tricritical behavior with cubic symmetry. We describe the translation of the tricritical model in a Coulomb gas. The results are verified numerically by means of transfer-matrix calculations. We use a generalized ADE model as an intermediary and present the expression of the one-point distribution function in that language. The analytic calculations are done both for the square and the honeycomb lattice.

A critical dilute O(n) model on the kagome lattice is investigated analytically and numerically. We employ a number of exact equivalences which, in a few steps, link the critical O(n) spin model on the kagome lattice to the exactly solvable critical q-state Potts model on the honeycomb lattice with q=(n+1)². The intermediate steps involve the random-cluster model on the honeycomb lattice and a fully packed loop model with loop weight n^′=sqrt[q] and a dilute loop model with loop weight n, both on the kagome lattice. This mapping enables the determination of a branch of critical points of the dilute O(n) model, as well as some of its critical properties. These properties differ from those of the generic O(n) critical points. For n=0, our model reproduces the known universal properties of the θ point describing the collapse of a polymer. For n≠0 it displays a line of multicritical points, with the same universal behavior as a branch of critical points that was found earlier in a dilute O(n) model on the square lattice. These findings are supported by a finite-size-scaling analysis in combination with transfer-matrix calculations.

The stability of multielectron bubbles in liquid helium is investigated theoretically. We find that multielectron bubbles are unstable against fission whenever the pressure is positive. It is shown that for moving bubbles the Bernoulli effect can result in a range of pressures over which the bubbles are stable.

Spontaneous emission of an excited two-level nonrelativistic atom is examined in the Schrödinger picture by treating the atom’s external and internal degrees of freedom on the same quantum footing. After all atomic transitions that lead to the evolution of this atom-vacuum system are analyzed, it is found that the atom’s spontaneous emission rate is reduced by its center-of-mass motion. It is also found that, as a result of the entanglement between atomic and photonic states, the spontaneous emission process cannot be explained as a process that takes place in one coordinate system moving with the atom but is observed in another stationary system.

We report molecular dynamics simulations of the recently discovered superelongation of carbon nanotubes (CNTs) at high temperatures. The nearly simultaneous activation and wide distribution of a large number of defects near the elastic limit play a key role in impeding the formation of localized predominant instability and facilitating large tensile elongation. It suggests new and more complex mechanisms for CNT superelongation in contrast with the previously proposed ideal defect glide and pseudoclimb. Defect interaction and evolution generate multistage necking and kinking and new types of larger defects that dominate the tensile elongation and breaking process. Intricate interplay between CNT sizes and defect nucleation and motion determine the overall deformation pattern.

We experimentally measure the kinetic energy and angular distributions of fragment ions of N₂ as a function of 820  nm femtosecond laser intensity. The angular dependence of fragments directly maps to the orbital symmetry of transient molecular ions. According to the observed energy and angular distribution of different charged fragment ions, reasonable fragmentation channels are proposed. Both the asymmetric channel (3, 1) and the symmetric channel (2, 2) are supposed to be generated directly from (2, 1) by different ionization mechanisms.

Systematic ab initio calculations show that the energy gap of BN nanoribbons (BNNRs) with zigzag or armchair edges can be significantly reduced by a transverse electric field and be completely closed at a critical field which decreases with increasing ribbon width. In addition, a distinct gap modulation in the ribbons with zigzag edges is presented when a reversed electric field is applied. In a weak field, the gap reduction of the BNNRs with zigzag edges originates from the field-induced energy level shifts of the spatially separated edge states, while the gap reduction of the BNNRs with armchair edges arises from the Stark effect. As the field gets stronger, the energy gaps of both types of the BNNRs gradually close due to the field-induced motion of nearly free electron states. Without the applied fields, the energy gap modulation by varying ribbon width is rather limited.

In this work, decoherence of a system from an entangled state is studied theoretically by computing the time-dependent probability amplitude with which the system remains in that state. The system is composed of, in addition to vacuum, identical two-level atoms that interact indirectly with each other through photons and have a uniform distribution in the vacuum. Under the assumption that in the initial entangled state only one atom is allowed to be in the excited state, a general expression for the probability amplitude is obtained and applied to two specific entangled states, one symmetric and one antisymmetric, to demonstrate that the system decoheres from these two states at the same rate under certain conditions.

The successive dynamical evolution of a Bose-Einstein condensate confined in a cylindrical well is numerically studied in the framework of the time-dependent Gross-Pitaevskii equation. Interference in the nonlinear matter wave leads to concentric density rings. The phase distribution exhibits a discontinuous sequence of plateaulike belts. Abrupt jumps in the phase between adjacent belts imply large radial superfluid velocity at the borderline. This, however, does not mean large particle current because the corresponding superfluid density is nearly zero. The density zeros along with the large gradient are identified as ring dark solitons, which have a brief lifetime before evolving into other soliton states.

Recently discovered ultralow friction (superlubricity) between incommensurate graphitic layers has raised great interest in understanding the interlayer interaction between graphene sheets under various physical conditions. In this work, we have studied the effects of interlayer distance change and in-sheet defects in modifying the interlayer friction in graphene sheets by extensive molecular-force-field statics calculations. The interlayer friction between graphene sheets with commensurate or incommensurate interlayer stacking increases with decreasing interlayer distance, but in the case of incommensurate stacking, ultralow friction can exist in a significantly expanded range of interlayer distance. The ultralow interlayer friction in the incommensurate stacking sheets is insensitive to the in-sheet defect of vacancy at a certain orientation. These results provide knowledge for possibly controlling friction between graphene sheets and offer insight into their applications.

Kondo resonances are a very precise measure of spin-polarized transport through magnetic impurities. However, the Kondo temperature, indicating the thermal range of stability of the magnetic properties, is very low. By contrast, we find for iron phthalocyanine a Kondo temperature in spectroscopic measurements which is well above room temperature. It is also shown that the signal of the resonance depends strongly on the adsorption site of the molecule on a gold surface. Experimental data are verified by extensive numerical simulations, which establish that the coupling between iron states and states of the substrate depends strongly on the adsorption configuration.

The longitudinal momentum distribution (P_//) of fragments after one-proton removal from ²³Al and reaction cross sections (σ_R) for ^23,24Al on a carbon target at 74A MeV have been measured. The ^23,24Al ions were produced through projectile fragmentation of 135A MeV ²⁸Si primary beam using the RIPS fragment separator at RIKEN. P_// is measured by a direct time-of-flight (TOF) technique, while σ_R is determined using a transmission method. An enhancement in σ_R is observed for ²³Al compared with ²⁴Al. The P_// for ²²Mg fragments from ²³Al breakup has been obtained for the first time. FWHM of the distributions has been determined to be 232 ± 28 MeV/c. The experimental data are discussed by using the Few-Body Glauber model. Analysis of P_// demonstrates a dominant d-wave configuration for the valence proton in ground state of ²³Al, indicating that ²³Al is not a proton halo nucleus.