Fano lineshape and phonon softening in single isolated metallic carbon nanotubes

Evolution of G-band modes of single metallic carbon nanotubes with the Fermi level shift is examined by simultaneous Raman and electron transport studies. Narrow Lorentzian line shape and upshifted frequencies are observed near the van Hove singularities. However, all G modes soften and broaden at the band crossing point. The concurrent appearance of an asymmetric Fano line shape at this point indicates that phonon-continuum coupling is intrinsic to single metallic tubes. The apparent Lorentzian line shapes of as-synthesized metallic tubes are induced by O2 adsorption causing the Fermi level shift.     

Dynamics of quantum noise in a tunnel junction under ac excitation

We report the first measurement of the dynamical response of shot noise (measured at frequency omega) of a tunnel junction to an ac excitation at frequency omega0. The experiment is performed in the quantum regime, variant Planck's over 2piomega approximately variant Planck's over 2piomega0>kBT at very low temperature T=35 mK and high frequency omega0/2pi=6.2 GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.     

Effective nucleon mass in deformed nuclei

The coupling of vibrations to nucleons moving in levels lying close to the Fermi energy of deformed rotating nuclei is found to lead to a number of effects: (i) shifts of the single-particle levels of the order of 0.5 MeV towards the Fermi energy and thus to an increase of the level density, (ii) single-particle state depopulation of the order of 30%, and thus spectroscopic factors approximately 0.7, etc. These effects, which we have calculated for 168Yb, can be expressed in terms of an effective mass, the so-called omega mass ( m(omega)), which is approximately 40% larger than the bare nucleon mass in the ground state. It is found that m(omega) displays a strong dependence with rotational frequency, eventually approaching the bare mass for Planck's over 2piomega(rot) approximately 0.5-0.6 MeV.     

Thickness and stacking geometry effects on high frequency overtone and combination Raman modes of graphene

We investigate two high frequency Raman overtone and combination modes of graphene named 2D' and 2D + G bands, and located at ~3240 and ~ 4260 cm^–1, respectively. The graphene thickness and stacking geometry effects for these two modes are systematically studied. The features of the 2D' band, which arises from intravalley double resonance, are not sensitive to the variation of thickness with single Lorentzian peak and fixed linewidth. We explain it theoretically by calculating the phonon dispersion mode in k‐space and find that the flat band region of longitudinal optical phonon near Γ point is the mechanism leading to the 2D' band nonsplit. With the thickness increasing, the band position exhibits blueshift and the linewidth increases for the 2D + G band. With changing thickness and stacking geometry of graphene, the intensities of these two high‐frequency bands show obvious different evolution compared with that of G band. Copyright © 2012 John Wiley & Sons, Ltd.     

Alkali-metal-induced topological nodal line semimetalin layered XN2 (X= Cr,Mo, W)

Based on first principles calculations and the K·p effective model, we propose that alkali metal deposition on the surface of hexagonal XN₂ (X= Cr, Mo, W) nanosheets induces topologically nontrivial phases in these systems. When spin orbit coupling (SOC) is disregarded, the electron-like conduction band from N-p_z orbitals can be considered to cross the hole-like valence band from X-d²_z orbitals, thereby giving rise to a topological nodal line state in lithium-functionalized XN₂ sheets (Li₂MoN₂ and Li₂WN₂). Such band crossing is protected by the existence of mirror reflection and time reversal symmetry. More interestingly, the bands cross exactly at the Fermi level, and the linear dispersion regions of such band crossings extend to as high as 0.9 eV above the crossing. For Li₂CrN₂, the results reveal the emergence of a Dirac cone at the Fermi level. Our calculations show that lattice compression decreases the thickness of a Li₂CrN₂ nanosheet, leading to phase transition to a nodal line semimetal. The evolution of the band gap of Li₂XN₂ at the Γ point indicates that the nontrivial topological character of Li₂XN₂ nanolayers is stable over a large strain range. When SOC is included, the band crossing point is gapped out giving rise to quantum spin Hall states in Li₂CrN₂ nanosheets, while for Li₂MoN₂, the SOC-induced gap at the crossing points is negligible.     

Br掺杂对石墨烯带隙影响的实验研究

石墨烯独特的结构和性能使其在纳米电子、半导体器件等领域成为研究的热点,但其零带隙的特性严重限制了其应用.采用化学气化沉积法制备了多层石墨烯,并使用溴蒸汽对制备的多层石墨烯进行掺杂,分析研究了溴蒸汽化学掺杂对石墨烯带隙的影响.为了对比溴蒸汽掺杂对石墨烯带隙的影响,使用633 nm He-Ne光分别测量了石墨烯掺杂前和掺杂后的拉曼光谱,根据拉曼光谱计算了石墨烯费米能级移动与掺杂溴蒸汽之间的关系.实验结果表明:溴蒸汽掺杂对石墨烯拉曼光谱G带产生影响;随着掺杂溴蒸汽体积的增加,拉曼光谱G带向高频移动并逐渐趋于稳定;G带和2D带强度比也迅速增加,并最终趋于稳定.费米能级的移动与G峰位置成线性关系,利用G峰峰值位置与费米能级实验关系式计算了溴掺杂后石墨烯的费米能级,分析了化学掺杂对石墨烯带隙的影响.     

Electric field effect tuning of electron-phonon coupling in graphene

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.     

Coulomb crystals in the harmonic lattice approximation

The dynamic structure factor &Stilde;(k,omega) and the two-particle distribution function g(r,t) of ions in a Coulomb crystal are obtained in a closed analytic form using the harmonic lattice (HL) approximation which takes into account all processes of multiphonon excitation and absorption. The static radial two-particle distribution function g(r) is calculated for classical (T greater, similarPlanck's over 2piomega(p), where omega(p) is the ion plasma frequency) and quantum (T相似文献   

Probing pauli blocking factors in quantum pumps with broken time-reversal symmetry

A recently demonstrated quantum electron pump is discussed within the framework of photon-assisted tunneling. Because of lack of time-reversal symmetry, different results are obtained for the pump current depending on whether or not final-state Pauli blocking factors are used when describing the tunneling process. While in both cases the current depends quadratically on the driving amplitude for moderate pumping, a marked difference is predicted for the temperature dependence. With blocking factors the pump current decreases roughly linearly with temperature until k(B)T approximately Planck's over 2piomega is reached, whereas without them it is unaffected by temperature, indicating that the entire Fermi sea participates in the transport.     

Interaction of optical phonons with anisotropic imperfections

Inelastic (Raman) light scattering by phonons interacting with anisotropic imperfections is investigated. Three different kind of disorder-induced defects (point, linear and planar) have been considered. The optical phonon width and line shape are found to depend importantly on the dimension of the imperfections. There is a close correspondence between the scale of the imperfection and the phonon line shape observed in the Raman scattering experiments. The dependence of the phonon frequency shift and width on the defect concentrations is calculated, and the critical concentrations at which the optical phonon can no longer be observed are determined. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 817–822 (25 December 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Topology of triple-point metals

We discuss and illustrate the appearance of topological fermions and bosons in triple-point metals where a band crossing of three electronic bands occurs close to the Fermi level.Topological bosons appear in the phonon spectrum of certain triple-point metals, depending on the mass of atoms that form the binary triple-point metal.We first provide a classification of possible triple-point electronic topological phases possible in crystalline compounds and discuss the consequences of these topological phases, seen in Fermi arcs, topological Lifshitz transitions, and transport anomalies.Then we show how the topological phase of phonon modes can be extracted and proven for relevant compounds.Finally,we show how the interplay of electronic and phononic topologies in triple-point metals puts these metallic materials into the list of the most efficient metallic thermoelectrics known to date.     

How does the substrate affect the Raman and excited state spectra of a carbon nanotube?

We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended across a trench and partially supported by a SiO₂-substrate. By tuning the laser excitation energy across the E ₃₃ excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial breathing mode (RBM), the D mode and the G mode show strong resonance enhancement of up to three orders of magnitude. In the supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E ₃₃ excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G band and D band is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect density in the supported CNT segments. By comparing the E ₃₃ resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements, however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser excitation power.     

Intrinsic spin Hall effect in platinum: first-principles calculations

Spin Hall effect (SHE) is studied with first-principles relativistic band calculations for platinum, which is one of the most important materials for metallic SHE and spintronics. We find that intrinsic spin Hall conductivity (SHC) is as large as approximately 2000(variant Planck's over 2 pi/e)(Omega cm)(-1) at low temperature and decreases down to approximately 200(variant Planck's over 2 pi/e)(Omega cm)(-1) at room temperature. It is due to the resonant contribution from the spin-orbit splitting of the doubly degenerated d bands at high-symmetry L and X points near the Fermi level. By modeling these near degeneracies by an effective Hamiltonian, we show that SHC has a peak near the Fermi energy and that the vertex correction due to impurity scattering vanishes. We therefore argue that the large SHE observed experimentally in platinum is of intrinsic nature.     

Nonadiabatic Kohn anomaly in a doped graphene monolayer

We compute, from first principles, the frequency of the E(2g), Gamma phonon (Raman G band) of graphene, as a function of the charge doping. Calculations are done using (i) the adiabatic Born-Oppenheimer approximation and (ii) time-dependent perturbation theory to explore dynamic effects beyond this approximation. The two approaches provide very different results. While the adiabatic phonon frequency weakly depends on the doping, the dynamic one rapidly varies because of a Kohn anomaly. The adiabatic approximation is considered valid in most materials. Here, we show that doped graphene is a spectacular example where this approximation miserably fails.     

Universal properties of a trapped two-component fermi gas at unitarity

We treat the trapped two-component Fermi system, in which unlike fermions interact through a two-body short-range potential having no bound state but an infinite scattering length. By accurately solving the Schr?dinger equation for up to N=6 fermions, we show that no many-body bound states exist other than those bound by the trapping potential, and we demonstrate unique universal properties of the system: Certain excitation frequencies are separated by 2variant Planck's over 2piomega, the wave functions agree with analytical predictions and a virial theorem is fulfilled. Further calculations up to N=30 determine the excitation gap, an experimentally accessible universal quantity, and it agrees with recent predictions based on a density functional approach.     

Exact removal of the center-of-mass spurious states from level densities

We give recursive formulae for the exact removal of the contribution of the center-of-mass spurious states from the fixed-spin and parity nuclear level density found in shell-model calculations, provided the total level density for restricted configurations is known. The method is valid for a large class of problems using a harmonic oscillator basis. Using our earlier methods based on statistical spectroscopy that utilize the centroids and widths for a restricted class of fixed-spin configurations, such as Nvariant Planck's over 2piomega excitations, one can calculate very accurately level densities free of spurious states. The approach is applicable to other fermion and boson systems trapped by an oscillator potential.     

Spin resonance in the d-wave superconductor CeCoIn5

Neutron scattering is used to probe antiferromagnetic spin fluctuations in the d-wave heavy fermion superconductor CeCoIn5 (T_(c)=2.3 K). Superconductivity develops from a state with slow (variant Planck's over 2piGamma=0.3+/-0.15 meV) commensurate [Q_(0)=(1/2,1/2,1/2)] antiferromagnetic spin fluctuations and nearly isotropic spin correlations. The characteristic wave vector in CeCoIn5 is the same as CeIn3 but differs from the incommensurate wave vector measured in antiferromagnetically ordered CeRhIn5. A sharp spin resonance (variant Planck's over 2piGamma<0.07 meV) at variant Planck's over 2piomega=0.60+/-0.03 meV develops in the superconducting state removing spectral weight from low-energy transfers. The presence of a resonance peak is indicative of strong coupling between f-electron magnetism and superconductivity and consistent with a d-wave gap order parameter satisfying Delta(q+Q0)=-Delta(q).     

InGaN薄膜的紫外共振喇曼散射

利用325nm紫外光激发,对不同组分的In_xGa_1-xN薄膜的喇曼散射谱进行了研究.在光子能量大于带隙的情况下,观察到显著增强的二阶A₁(LO)声子散射峰.二阶LO声子峰都从一阶LO声子的二倍处向高能方向移动,移动量随样品In组分的增加而增大,认为是带内Frhlich相互作用决定的多共振效应引起的.分析了一阶LO声子散射频率和峰型与In组分的关系.在喇曼谱中观察到样品存在相分离现象,并与X射线衍射的实验结果进行 关键词： xGa_1-xN合金')" href="#">In_xGa_1-xN合金 紫外共振喇曼散射 二阶声子 相分离     

Antibunched photons emitted by a quantum point contact out of equilibrium

Motivated by the experimental search for "GHz nonclassical light," we identify the conditions under which current fluctuations in a narrow constriction generate sub-Poissonian radiation. Antibunched electrons generically produce bunched photons, because the same photon mode can be populated by electrons decaying independently from a range of initial energies. Photon antibunching becomes possible at frequencies close to the applied voltage V x e/variant Planck's over 2pi, when the initial energy range of a decaying electron is restricted. The condition for photon antibunching in a narrow frequency interval below eV/variant Planck's over 2pi reads [SigmanTn(1-Tn)]2<2Sigman[Tn(1-Tn)]2, with Tn an eigenvalue of the transmission matrix. This condition is satisfied in a quantum point contact, where only a single Tn differs from 0 or 1. The photon statistics is then a superposition of binomial distributions.