Electric-field effect on the angle-dependent magnetotransport properties of quasi-one-dimensional conductors

We report a novel electric field effect on angular dependent magnetotransport in quasi-one-dimensional layered conductors with a pair of sheetlike Fermi surfaces. Under tilted magnetic fields and additional interlayer electric fields, semiclassical electron orbits on two Fermi sheets become periodic at different magnetic field orientations. This causes double splitting of the Lebed's commensurability resonance in interlayer transport, and the amount of splitting allows us to estimate the Fermi velocity directly. We have successfully demonstrated this effect in the organic conductor alpha-(BEDT-TTF)2KHg(SCN)4.     

Direct measurement of the Fermi energy in graphene using a double-layer heterostructure

We describe a technique which allows a direct measurement of the relative Fermi energy in an electron system by employing a double-layer heterostructure. We illustrate this method by using a graphene double layer to probe the Fermi energy as a function of carrier density in monolayer graphene, at zero and in high magnetic fields. This technique allows us to determine the Fermi velocity, Landau level spacing, and Landau level broadening. We find that the N=0 Landau level broadening is larger by comparison to the broadening of upper and lower Landau levels.     

Landau levels in graphene in the presence of emergent gravity

We consider graphene in the presence of external magnetic field and elastic deformations that cause emergent magnetic field. The total magnetic field results in the appearance of Landau levels in the spectrum of quasiparticles. In addition, the quasiparticles in graphene experience the emergent gravity. We consider the particular choice of elastic deformation, which gives constant emergent magnetic field and vanishing torsion. Emergent gravity may be considered as perturbation. We demonstrate that the corresponding first order approximation affects the energies of the Landau levels only through the constant renormalization of Fermi velocity. The degeneracy of each Landau level receives correction, which depends essentially on the geometry of the sample. There is the limiting case of the considered elastic deformation, that corresponds to the uniformly stretched graphene. In this case in the presence of the external magnetic field the degeneracies of the Landau levels remain unchanged.     

基于光子晶体异质结构的磁可调石墨烯多带吸收

石墨烯在光电子学领域具有广泛应用,但石墨烯的吸收率较低限制了其在某些方面的应用。为了改善单层石墨烯的吸收特性,在前人研究的基础上,设计了石墨烯和光子晶体异质结构构成的复合结构。利用4×4传输矩阵法研究了外磁场、费米能量和设计波长等参数对石墨烯吸收特性的影响。结果表明,所设计的光学结构使石墨烯既保持了原有的宽吸收带,还增加了数目可调的窄吸收带。由于考虑到磁光效应,石墨烯的吸收特性表现出一定的磁圆二色性。对于各吸收带,通常情况下左旋圆偏振光的吸收率要大于右旋圆偏振光的吸收率。但调节外磁场和费米能量,可使各吸收带具有99%以上的吸收,在一定的条件下,还可实现近完美的100%吸收。研究结果为光电子学领域中基于石墨烯的相关器件的设计提供了参考。     

Magnetoresistance of organic conductors in the vicinity of a topological phase transition

We analyze the effect of magnetic breakdown on the resistance of layered organic conductors with a multisheet Fermi surface consisting of a cylinder and two slightly corrugated planes along the projection of the momentum onto the normal to the layers. Analytic expressions are derived for the charge carrier distribution function, and the dependences of the interlayer and intralayer conductivities on the magnitude and orientation of the external magnetic field in the immediate vicinity of a topological phase transition are determined when the distance between the different sheets of the Fermi surface is quite small, but the topological structure of the Fermi surface is still intact.     

NMR investigation of how free composite fermions are at nu=1 / 2

NMR measurements of the electron spin polarization (P) have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature dependence of P to models of free and interacting composite fermions (CF), the imbalance of spin-up and spin-down CF Fermi seas is mapped as a function of Zeeman energy. Independent measurements of the CF effective mass, g factor, and Fermi energy are obtained from the thermal activation of P in tilted fields. The filling factor dependence of the P for 2 / 5相似文献   

Transport properties of Dirac electrons in graphene based double velocity-barrier structures in electric and magnetic fields

Using transfer matrix method, transport properties in graphene based double velocity-barrier structures under magnetic and electric fields are numerically studied. It is found that velocity barriers for the velocity ratio (the Fermi velocity inside the barrier to that outside the barrier) less than one (or for the velocity ratio greater than one) have properties similar to electrostatic wells (or barriers). The velocity barriers for the velocity ratio greater than one significantly enlarge the resonant tunneling region of electrostatic barriers. In the presence of magnetic field, the plateau width of the Fano factor with a Poissonian value shortens (or broadens) for the case of the velocity ratio less than one (or greater than one). When the Fermi energy is equal to the electrostatic barrier height, for different values of the velocity ratio, both the conductivities and the Fano factors remain fixed.     

Phonon-induced many-body renormalization of the electronic properties of graphene

We develop a theory for the electron-phonon interaction effects on the electronic properties of graphene. We analytically calculate the electron self-energy, spectral function, and the band velocity renormalization due to phonon-mediated electron-electron interaction, finding that phonon-mediated electron-electron coupling has a large effect on the graphene band structure renormalization. Our analytic theory successfully captures the essential features of the observed graphene electron spectra in the angle-resolved photoemission experiments, predicting a kink at approximately 200 meV below the Fermi level and a reduction of the band velocity by approximately 10-20% at the experimental doping level.     

Opening an energy gap in an electron double layer system at the integer filling factor in a tilted magnetic field

We employ magnetocapacitance measurements to study the spectrum of a double layer system with gate-voltage-tuned electron density distributions in tilted magnetic fields. For the dissipative state in normal magnetic fields at filling factor v=3 and 4, a parallel magnetic field component is found to give rise to the opening of a gap at the Fermi level. We account for the effect in terms of parallel-field-caused orthogonality breaking of the Landau wave functions with different quantum numbers for two subbands.     

Rippling and the external electric field on conductance in corrugated graphene nanoribbons molecular device

Molecular devices constructed using corrugated graphene nanoribbons (GNRs) are proposed in the paper. Recursive Green's function calculations show that the intrinsic ripples in graphene and the external electric field energy play important roles on the electron transport properties. Negative differential resistance is observed in zigzag corrugated GNRs. With the wavelength of the ripples decreasing, both the zigzag and armchair corrugated GNRs exhibit ON/OFF characteristics. On applying external electric field, current decreases dramatically in zigzag corrugated GNRs. These findings show that corrugated GNRs can be used to design functional nanoscale devices.     

Precession of an electron-magnetohydrodynamic field-reversed configuration

A field-reversed configuration is generated in a large laboratory plasma in the parameter regime of electron magnetohydrodynamics. During its free relaxation, the magnetic moment is observed to precess when tilted from its original axis. The precession velocity is the electron drift velocity in the toroidal current layer. The precession is a manifestation of frozen-in field lines in a moving electron fluid.     

Transport and thermodynamic properties of a 2DEG in the presence of tilted magnetic field: the influence of Rashba interaction

The effects of Rashba spin-orbit coupling on the physical properties of a non-interacting two-dimensional electron gas (2DEG) in presence of a tilted magnetic field are investigated. The spin-orbit and Zeeman interactions depend on the intensity and direction of the external magnetic field and we study their influence on the physical properties of the system. We obtain the energy spectrum, the density of states, the Fermi energy, and we analyze the combined effect of the Zeeman and Rashba Spin-Orbit Interaction on the transport and thermodynamic properties.     

Electron-hole pairing with nonzero momentum in a graphene bilayer

Electron-hole pairing due to the Coulomb interaction in the system of two graphene sheets has been considered. The critical transition temperature has been determined as a function of both the distance between the electron and hole Fermi lines and the triangular distortion of their spectrum. It has been shown that when the distance between Fermi lines is longer than a critical value, the temperature of the transition to a state with nonzero momentum of Cooper pairs (Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state) is higher than the temperature of the transition to the Bardeen-Cooper-Schrieffer state. The Josephson effect for the FFLO state has been analyzed, which is due to the tunneling of charge carriers between the graphene sheets. It has been shown that the spatial structure of the order parameter of the system in this state can be reconstructed, i.e., the FFLO state can be identified from the dependence of the tunneling current on the magnetic field parallel to the graphene sheets. Other experimental methods for studying the phase diagram of the system have been discussed.     

The effects of the magnetopolaron on the energy gap opening in graphene

The magnetopolaron is formed via electron-acoustic deformation phonon coupling in the presence of a magnetic field in monolayer graphene. We find that an energy gap (EG) is opened due to the electron-phonon coupling. Both linear and square-root forms for the dependence of the EG on the magnetic field are obtained, which are in agreement with experimental measurements. Furthermore, we suggest that the EG can be estimated through observing the variation of Fermi velocity in cyclotron resonance experiments. The relation of the EG with the Debye cut-off wavenumber is also discussed.     

Transport in magnetic graphene superlattice with Rashba spin-orbit interaction

Based on the transfer-matrix method, we have investigated the spin-dependent transport properties of magnetic graphene superlattice in the presence of Rashba spin-orbit interaction (RSOI). It is shown that the angular range of the spin transmission probability through magnetic graphene superlattice can be efficiently controlled by the number of barriers. As the number of magnetic barriers increases, the angular range of the transmission through the magnetic superlattice decreases, the gaps in the transmission and conductivity versus energy become wider. It is also found that the spin conductivities oscillate with the Fermi energy and RSOI strength. Specifically, when a magnetic field is present, the spin polarisation can be observed, whereas for the RSOI alone it is zero. Application of such a phenomenon to design a spin polarised electron device based on the graphene material is anticipated.     

Transverse field effect in graphene ribbons

It is shown that a graphene ribbon, a ballistic strip of carbon monolayer, may serve as a quantum wire whose electronic properties can be continuously and reversibly controlled by an externally applied transverse voltage. The electron bands of armchair-edge ribbons undergo dramatic transformations: The Fermi surface fractures, Fermi velocity and effective mass change sign, and excitation gaps are reduced by the transverse field. These effects are manifest in the conductance plateaus, van Hove singularities, thermopower, and activated transport. The control over one-dimensional bands may help enhance effects of electron correlations, and be utilized in device applications.     

Local flattening of the Fermi surface and quantum oscillations in the magnetoacoustic response of a metal

In the present work, we theoretically analyze the effect of the Fermi surface local geometry on quantum oscillations in the velocity of an acoustic wave travelling in metal across a strong magnetic field. We show that local flattenings of the Fermi surface could cause significant amplification of quantum oscillations. This occurs due to enhancement of commensurability oscillations modulating the quantum oscillations in the electron density of states on the Fermi surface. The amplification in the quantum oscillations could be revealed at fitting directions of the magnetic field.     

Hydrazine reduced exfoliated graphene/graphene oxide layers and?magnetoconductance measurements of Ge-supported graphene layers

Inexpensive production and characteristic magnetoconductance fluctuation of Ge-supported stable graphene/graphene oxide layers are being reported. The changeover from graphite to graphene oxide structure during synthesis was evident from X-ray diffraction patterns whereas development of mono and bilayer graphene/graphene oxide was confirmed by electron microscopy studies. Responding to applied magnetic fields (up to 0.3 T), the Ge-supported graphene layers are shown to exhibit prominent magnetoconductance steps and are attributed to the alteration of Landau levels across the Fermi surface. While low-cost fabrication process is attractive for large scale production, the advantage of short synthesis time and understanding magneto-transport mechanism would find relevance in graphene-based nanodevices and circuits.     

Mesoscopic conductance fluctuations in graphene

We study fluctuations of the conductance of micron-sized graphene devices as a function of the Fermi energy and magnetic field. The fluctuations are studied in combination with analysis of weak localization which is determined by the same scattering mechanisms. It is shown that the variance of conductance fluctuations depends not only on inelastic scattering that controls dephasing but also on elastic scattering. In particular, contrary to its effect on weak localization, strong intervalley scattering suppresses conductance fluctuations in graphene. The correlation energy, however, is independent of the details of elastic scattering and can be used to determine the electron temperature of graphene structures.     

Engineering the electronic structure of graphene superlattices via Fermi velocity modulation

Graphene superlattices have attracted much research interest in the last years, since it is possible to manipulate the electronic properties of graphene in these structures. It has been verified that extra Dirac points appear in the electronic structure of the system. The electronic structure in the vicinity of these points has been studied for a gapless and gapped graphene superlattice and for a graphene superlattice with a spatially modulated energy gap. In each case a different behavior was obtained. In this work we show that via Fermi velocity engineering it is possible to tune the electronic properties of a graphene superlattice to match all the previous cases studied. We also obtained new features of the system never observed before, reveling that the electronic structure of graphene is very sensitive to the modulation of the Fermi velocity. The results obtained here are relevant for the development of novel graphene-based electronic devices.