Electronic transport spectroscopy of carbon nanotubes in a magnetic field

We report magnetic field spectroscopy measurements in carbon nanotube quantum dots exhibiting fourfold shell structure in the energy level spectrum. The magnetic field induces a large splitting between the two orbital states of each shell, demonstrating their opposite magnetic moment and determining transitions in the spin and orbital configuration of the quantum dot ground state. We use inelastic cotunneling spectroscopy to accurately resolve the spin and orbital contributions to the magnetic moment. A small coupling is found between orbitals with opposite magnetic moment leading to anticrossing behavior at zero field.     

Ultrashort pulse propagation in carbon nanotubes in a magnetic field

The problem of ultra-short optical pulse behavior in a system of carbon nanotubes with an applied magnetic field parallel to the nanotube axis was considered. The electromagnetic field was explored using the Maxwell equations. The electronic system of the carbon nanotubes was a quantum system and was mechanically investigated for the case of low temperatures. The distributional pattern of the ultra short pulses and their collision were established by means of numerical modeling.     

Peierls相变与磁场中碳纳米管的场发射

应用紧束缚模型和WKB方法研究碳纳米管的out-of-plane型Peierls相变,及其对碳纳米管的场发射的影响.结果发现Peierls相变会在室温出现,并使碳纳米管费米面附近出现能隙,导致碳纳米管发生金属—半导体转变,从而抑制碳纳米管的场发射.磁场也会抑制Peierls形变,Peierls相变和磁场相互竞争影响碳纳米管的能带结构,从而影响碳纳米管的场发射. 关键词： 场发射 碳纳米管 Peierls相变     

Conductivity of single-walled carbon nanotubes

In a system of N interacting single-level quantum dots (QDs), we study the relaxation dynamics and the current–voltage characteristics determined by symmetry properties of the QD arrangement. Different numbers of dots, initial charge configurations, and various coupling regimes to reservoirs are considered. We reveal that effective charge trapping occurs for particular regimes of coupling to the reservoir when more than two dots form a ring structure with the C_N spatial symmetry. We reveal that the effective charge trapping caused by the C_N spatial symmetry of N coupled QDs depends on the number of dots and the way of coupling to the reservoirs. We demonstrate that the charge trapping effect is directly connected with the formation of dark states, which are not coupled to reservoirs due to the system spatial symmetry C_N. We also reveal the symmetry blockade of the tunneling current caused by the presence of dark states.     

The density of states for carbon nanotubes in a uniform magnetic field

Dispersion laws for carbon nanotubes in a uniform magnetic field are obtained in an explicit form in a zero-range-potential model. The band structure of the spectrum is studied, and the density of states is calculated numerically.     

Design of soliton electron lattices in carbon nanotubes by a magnetic field

Electron states in the conduction band of carbon nanotubes are studied in the presence of a constant magnetic field perpendicular to the tube axis within the framework of Hubbard's model. Numerical solutions describing electron density waves corresponding to soliton lattices are obtained. The possibility of control over the electron lattice parameters with the help of a constant magnetic field is established. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 17–23, December, 2008.     

The nonlinear optical properties of nanotubes with spiral defects in a longitudinal magnetic field

It is demonstrated that the anisotropic transfer of photon momentum to an electronic subsystem results in induction of a photon-drag EMF in a standing electromagnetic wave along the axis of a nanotube with a spiral defect, which confirms the assumption found in the literature that the occurrence of such an effect in the presence of an external magnetic field is possible not only in 2-D systems but also in nanotubes with a spiral symmetry. One of the potential mechanisms of inducing the EMF connected with the spatial asymmetry of the electron-phonon interaction in a nanotube with a spiral defect is considered. This mechanism allows for such an EMF to occur upon heating the electron system by the Joule heat of the photon-drag current that flows through the nanotube.     

Conductivity of two-component systems in a magnetic field

The influence of a magnetic field on the effective conductivity of a system with a chessboard structure is studied under conditions of the Hall effect. It is shown that in this case a new physical effect occurs, involving an oscillatory dependence of the charge density at the interfaces on passage through the bifurcation point. The system possesses considerable magnetoresistance.     

Extremely short electromagnetic pulses in an array of carbon nanotubes with a longitudinal field inhomogeneity

The propagation of bipolar electromagnetic pulses in an array of semiconductor carbon nanotubes has been investigated. The inhomogeneity of the pulse field along the axis of the nanotubes has been taken into account for the first time. The evolution of the electromagnetic field and charge density in the sample has been described by the set of Maxwell’s equations and the continuity equation. The possibility of stable propagation of bipolar electromagnetic pulses occurring in an array of nanotubes has been demonstrated by numerical simulation. It has been shown that the propagation of the electromagnetic pulses induces the redistribution of the electron density in the sample.     

Absolute negative conductivity in zig-zag carbon nanotubes in the presence of a magnetic field

The volt-ampere and gauss-ampere characteristics of zig-zag carbon nanotubes are calculated at low temperatures based on the average electron method. The characteristics obtained are analyzed as functions of the magnetic field strength. It is revealed that a constant electric field can arise spontaneously in carbon nanotubes upon application of an alternating high-frequency electric field. This effect can be due to the nonequilibrium electron subsystem of the carbon nanotubes.     

Optical properties of semiconducting carbon nanotubes under axial magnetic field

The linear polarizability absorption spectra of semiconducting carbon nanotubes under axial magnetic field (B) have been calculated by the π-orbital tight-binding model and sum-over-state method. We have found that the optical spectra are split by the B-induced symmetry breaking and the amount of splitting increases with increase of magnetic field. Although the results are obtained within the noninteracting tight-binding model, the amount of splitting is still consistent with the experimental observation, offering a fast estimation of the B-induced splitting. Our numerical results also indicate that the splitting amounts of the second and third absorption peaks are close to that of the first one, which may be observed by the future experiments.     

Control of soliton lattices of Hubbard electrons in carbon nanotubes by a magnetic field

On the basis of the Hubbard model for electrons in the conduction band of carbon nanotubes, the numerical solutions describing soliton lattices are obtained, and the possibility of controlling the lattice parameters using a dc magnetic field is established.     

Three-dimensional few-cycle optical pulses in an array of carbon nanotubes inside a magnetic field

The behavior of three-dimensional few-cycle optical pulses, propagating in a system of carbon nanotubes inside an external magnetic field applied parallel to the nanotube axes and perpendicular to the direction of pulse propagation, is studied. The evolution of the electromagnetic field is classically derived using Maxwell’s equations.     

磁场中碳纳米管电子结构的紧束缚法研究

利用石墨平面碳原子轨道作sp2杂化时π电子的紧束缚模型,对磁场中直状单层碳纳米管(SWNTs)的电子结构进行理论推导和分析。磁场对碳纳米管的波矢产生影响,从而使碳纳米管的电子结构及能隙均以磁通量子Φ0(=h/e)为周期随磁通量Φ周期性变化。     

Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB₃ (MmNi₃, MmFe_1.5Ni_1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm² at 0.9 V/μm.     

Vibration response of double-walled carbon nanotubes subjected to an externally applied longitudinal magnetic field: A nonlocal elasticity approach

The magnetic properties of carbon nanotubes and their mechanical behaviour in a magnetic field have attracted considerable attention among the scientific and engineering communities. This paper reports an analytical approach to study the effect of a longitudinal magnetic field on the transverse vibration of a magnetically sensitive double-walled carbon nanotube (DWCNT). The study is based on nonlocal elasticity theory. Equivalent analytical nonlocal double-beam theory is utilised. Governing equations for nonlocal transverse vibration of the DWCNT under a longitudinal magnetic field are derived considering the Lorentz magnetic force obtained from Maxwell's relation. Numerical results from the model show that the longitudinal magnetic field increases the natural frequencies of the DWCNT. Both synchronous and asynchronous vibration phases of the tubes are studied in detail. Synchronous vibration phases of DWCNT are more affected by nonlocal effects than asynchronous vibration phases. The effects of a longitudinal magnetic field on higher natural frequencies are also presented. Vibration response of DWCNT with outer-wall stationary and single-walled carbon nanotube under the effect of longitudinal magnetic field are also discussed in the paper.     

Conductivity of a semiconductor superlattice in a transverse magnetic field

The conductivity of a semiconductor superlattice in a magnetic field perpendicular to the superlattice axis is calculated. The dependence of the conductivity on the magnetic field strength is studied. It is found that there is a magnetic-field range where the resistivity is proportional to the magnetic field strength.     

Effect of an external magnetic field on Landau damping in carbon nanotubes

A quantum-mechanical formula for describing the dependence of the longitudinal permittivity of a nanotube on its characteristic parameters and the magnetic field strength was derived. The imaginary part of the permittivity was isolated, and the cases of a Boltzmann and degenerate electron gas were examined. For a degenerate electron gas, asymptotic expressions for absorption in the short- and long-wave ranges were obtained. It was demonstrated that the absorption coefficient for a longitudinal electromagnetic wave exhibits Aharonov-Bohm oscillations.     

Conductivity of 2D polycrystalline media in a magnetic field

The effective conductivity of 2D polycrystalline media in a magnetic field is considered. A current trap model constructed for strongly anisotropic polycrystalline media makes it possible to determine the distribution of the Joule heat liberated in such media.     

轴向磁场对碳纳米管电子性质的影响

用紧束缚法研究了单壁碳纳米管在轴向磁场下费米能级附近电子性质的变化规律.研究发现：能隙随轴向磁场变化的快慢及磁致能隙峰值都与碳纳米管直径有着紧密联系；对于相同直径的碳纳米管，金属性管的磁致能隙峰值最大.具体计算了锯齿型碳纳米管费米能级附近电子态密度随轴向磁场的变化关系,发现所有碳纳米管的电子态密度和能隙变化都体现出周期性.磁场使得碳纳米管发生绝缘体-金属周期性相转变的根本原因是由于在磁场的作用下范霍夫奇异点出现分裂-移动-融合的周期性变化. 关键词： 磁场 碳纳米管 紧束缚法 范霍夫奇异点