Multilayer Massless Dirac Fermion System Research Articles

Angle dependent interlayer magnetoresistance in multilayer graphene stacks

Interlayer magnetoresistance (ILMR) effect is explored in a vertical stack of weakly coupled multilayer graphene as grown by chemical vapor deposition. This effect has been characterized as a function of temperature and tilt angle of the magnetic field with respect to the interlayer current. To our knowledge, this is the first experimental report on angle dependent ILMR effect in graphitic systems. Our data agree qualitatively with the existing theories of ILMR in multilayer massless Dirac Fermion systems. However, a sharper change in ILMR has been observed as the tilt angle of the magnetic field is varied. A physical explanation of this effect is proposed, which is consistent with our experimental scenario.

Anisotropic spin motive force in multi-layered Dirac fermion system, α-(BEDT-TTF)2I3

We investigate the anisotropic spin motive force in α-(BEDT-TTF)2I3, which is a multi-layered massless Dirac fermion system under pressure. Assuming the interlayer antiferromagnetic interaction and the interlayer anisotropic ferromagnetic interaction, we numerically examine the spin ordered state of the ground state using the steepest descent method. The anisotropic interaction leads to the anisotropic spin ordered state. We calculate the spin motive force produced by the anisotropic spin texture. The result quantitatively agrees with the experiment.

Effect of Interlayer Spin-Flip Tunneling for Interlayer Magnetoresistance in Multilayer Massless Dirac Fermion Systems

We investigate the effect of the interlayer spin-flip tunneling for the interlayer magnetoresistance under magnetic fields in α-(BEDT-TTF)2I3, which is a multilayer massless Dirac fermion system under pressure. The mean field of the spin-flip correlation associated with the interlayer Coulomb interaction enables the interlayer spin-flip tunneling. Assuming the non-vertical interlayer spin-flip tunneling, we calculate the interlayer magnetoresistance using the Kubo formula. The crossover magnetic field, at which the interlayer magnetoresistance changes from positive to negative is shifted by the Zeeman energy and in good agreement with the experiment.

Spin-Ordered States in Multilayer Massless Dirac Fermion Systems

We investigate the spin-ordered states in multilayer massless Dirac fermion systems under magnetic fields, in which the intralayer interaction is ferromagnetic owing to the exchange interaction, while the interlayer interaction is antiferromagnetic arising from the interlayer hopping and the on-site Coulomb repulsion. The possible spin-ordered states are examined within the mean field theory, and we apply it to alpha-(BEDT-TTF)2I3, which is a multilayer massless Dirac fermion system under pressure. In the weak interlayer coupling regime the system exhibits a ferromagnetically spin-ordered state with the effective Zeeman g-factor less than two contrasting to that observed in the single-layer graphene.

Quantum Hall effect in multilayered massless Dirac fermion systems with tilted cones

We report the first observation of Shubnikov-de Haas (SdH) oscillations and quantized Hall resistance in the multilayered massless Dirac fermion system $\alpha$-(BEDT-TTF)$_2$I$_3$ with tilted cones. Holes were injected into the thin crystal fixed on a polyethylene naphthalate (PEN) substrate by contact electrification. The detection of SdH oscillations whose phase was modified by Berry's phase $\pi$ strongly suggested that the carrier doping was successful in this system. We succeeded in detecting the quantum Hall effect (QHE) with the steps which is the essence of two dimensional Dirac fermion systems. The number of effectively doped layers was examined to be two in this device. We reveal that the correlation between effective layers plays an important role in QHE.

Anomalous Thermoelectric Transport and Giant Nernst Effect in Multilayered Massless Dirac Fermion System

We have measured the thermopower and Nernst effect of the multilayered massless Dirac fermion system α-(BEDT-TTF)2I3 under pressure. In the quantum limit region, the extremely large Nernst effect rather than the Seebeck effect was observed. This unusual behavior results from the intrinsic nature of the zero-mode Landau level characteristic of the Dirac fermion system. The possible origin of the giant Nernst signal compared with that of graphene is the sharp width of the Landau level and the Zeeman gap in high-purity α-(BEDT-TTF)2I3 crystal.

Specific Heat of the Multilayered Massless Dirac Fermion System

We have measured the specific heat of the multilayered massless Dirac fermion system α-(BEDT-TTF) 2 I 3 under pressure. The temperature dependence of the specific heat is almost quadratic at low temperature, suggesting massless Dirac dispersion in this system. In a magnetic field, the specific heat shows anomalous behavior, which can be understood by assuming the zero-mode Landau level characteristic of the Dirac fermion system and its spin splitting. These results are in good agreement with the reported magnetoresistance.

Magnetotransport of Massless Dirac Fermions in Multilayer Organic Conductors

Magnetotransport has been considered in multilayer massless Dirac fermion systems, in which two-dimensional (2D) layers with a pair of Dirac-cone dispersion stack with weak interlayer coupling. The Fermi level is assumed to be fixed at the Dirac point resulting in the zero-gap conductor. At the high-field quantum limit, where only the n = 0 Landau level at the Dirac point is partially occupied, all elements of conductivity tensor have been evaluated by the lowest order contribution of interlayer coupling. We have deduced saturation of in-plane resistance, local maxima of in-plane Hall resistance, negative interlayer magnetoresistance, and cotθ-type unusual angle-dependence of interlayer Hall resistance. These results propose good explanations for mysterious transport features observed in organic zero-gap conductors, α-(BEDT-TTF)2I3 and its related compounds.

Anomalous Interlayer Hall Effect in Multilayer Massless Dirac Fermion System at the Quantum Limit

Magnetotransport features, particularly interlayer Hall effect, have been discussed in multilayer massless Dirac fermion systems, in which two-dimensional layers having Dirac-cone dispersion are stacked with weak interlayer coupling. At the quantum limit, where only the n =0 Landau level is partially occupied, the conductivity tensor has been evaluated by the lowest order contribution of interlayer coupling. In addition to in-plane transport behaviors, we have obtained the remarkable interlayer Hall effect which exhibits no dependence on field strength but anomalous dependence on field orientation. These results have successfully explained mysterious transport features observed in a zerogap organic conductor, α-(BEDT-TTF) 2 I 3 .

Spin and valley splittings in multilayered massless Dirac fermion system

The interlayer magnetoresistance in a multilayered massless Dirac fermion system, $\ensuremath{\alpha}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}{\text{I}}_{3}$, under hydrostatic pressure was investigated. We succeeded in detecting the zero-mode $(n=0)$ Landau level and its spin splitting in the magnetic field normal to the two-dimensional plane. We demonstrated that the effective Coulomb interaction in the magnetic field intensifies the spin splitting of zero-mode Landau carriers. At temperatures below 2 K, magnetic fields above several Tesla break the twofold valley degeneracy.

Crossover from Positive to Negative Interlayer Magnetoresistance in Multilayer Massless Dirac Fermion System with Non-Vertical Interlayer Tunneling

We present a theoretical description of the interlayer magnetoresistance in the layered Dirac fermion system with the application to the organic conductor α-(BEDT-TTF) 2 I 3 under pressure. Assuming a non-vertical interlayer tunneling and including higher Landau level effects we calculate the interlayer conductivity using the Kubo formula. We propose a physical picture of the experimentally observed crossover from the negative interlayer magnetoresistance, where the Dirac fermion zero energy Landau level plays a central role, to the positive interlayer magnetoresistance that is the consequence of the Landau level mixing effect upon non-vertical interlayer hopping. The crossover magnetic field depends on the Landau level broadening factor and can be used to determine the Dirac fermion Landau level energy spectrum.

Effect of the Zero-Mode Landau Level on Interlayer Magnetoresistance in Multilayer Massless Dirac Fermion Systems

We report on the experimental results of interlayer magnetoresistance in the multilayer massless Dirac fermion system alpha-(BEDT-TTF)2I3 under hydrostatic pressure and its interpretation. We succeeded in detecting the zero-mode Landau level (n=0 Landau level) that is expected to appear at the contact points of Dirac cones in the magnetic field normal to the two-dimensional plane. The characteristic feature of zero-mode Landau carriers including the Zeeman effect is clearly seen in the interlayer magnetoresistance.