Molecular Dirac Fermion Systems — Theoretical and Experimental Approaches —

Kajita, Kôji; Nishio, Y.; Tajima, Naoya; Suzumura, Yoshikazu; Kobayashi, Akito

doi:10.7566/jpsj.83.072002

Cited by 201 publications

(214 citation statements)

References 79 publications

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“…Layered organic conductors may offer an experimental platform for exploring conductivity scalings discussed in this paper [46][47][48]. Being soft, organic materials under an uniaxial pressure acquire substantial changes in hopping integrals among molecular orbitals, which sometimes result in a formation of a pair of two-dimensional gapless Dirac fermions in their elecrotonic band structures [46,47].…”

Section: Discussionmentioning

confidence: 99%

“…Being soft, organic materials under an uniaxial pressure acquire substantial changes in hopping integrals among molecular orbitals, which sometimes result in a formation of a pair of two-dimensional gapless Dirac fermions in their elecrotonic band structures [46,47]. Thereby, the in-plane uniaxial pressure continuously changes the location of the gapless points in the k-space, and it is predicted that the pair of the two Dirac points annihilate with each other above a certain critical pressure [46,47,49,50]. In such layered conductors, the out-of-plane uniaxial pressure can also enhance interlayer hopping strengths, which may lead to a formation and pair annihilation of three-dimensional gapless Dirac or Weyl points with or without the relativistic spin-orbit interaction [1,51].…”

Section: Discussionmentioning

confidence: 99%

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Quantum multicriticality in disordered Weyl semimetals

Luo

Ohtsuki

et al. 2018

Phys. Rev. B

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In electronic band structure of solid state material, two band touching points with linear dispersion appear in pair in the momentum space. When they annihilate with each other, the system undergoes a quantum phase transition from three-dimensional Weyl semimetal (WSM) phase to a band insulator phase such as Chern band insulator (CI) phase. The phase transition is described by a new critical theory with a 'magnetic dipole' like object in the momentum space. In this paper, we reveal that the critical theory hosts a novel disorder-driven quantum multicritical point, which is encompassed by three quantum phases, renormalized WSM phase, CI phase, and diffusive metal (DM) phase. Based on the renormalization group argument, we first clarify scaling properties around the band touching points at the quantum multicritical point as well as all phase boundaries among these three phases. Based on numerical calculations of localization length, density of states and critical conductance distribution, we next prove that a localization-delocalization transition between the CI phase with a finite zero-energy density of states (zDOS) and DM phase belongs to an ordinary 3D unitary class. Meanwhile, a localization-delocalization transition between the Chern insulator phase with zero zDOS and a renormalized Weyl semimetal (WSM) phase turns out to be a direct phase transition whose critical exponent ν = 0.80 ± 0.01. We interpret these numerical results by a renormalization group analysis on the critical theory.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Quantum multicriticality in disordered Weyl semimetals

Luo

Ohtsuki

et al. 2018

Phys. Rev. B

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“…[24][25][26] They predicted a zero-gap state under high pressure, where -in contrast to graphenethe Dirac points do not occur at high-symmetry points and can be tuned by pressure. Although the Dirac cone is anisotropic and tilted, 23,27 as illustrated in Fig. 1(c), the band dispersion might allow the observation of mass-less fermions.…”

Section: Introductionmentioning

confidence: 99%

Pressure-dependent optical investigations ofα−(BEDT-TTF)2I3: Tuning charge order and narrow gap towards a Dirac semimetal

et al. 2016

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Infrared optical investigations of α-(BEDT-TTF)2I3 have been performed in the spectral range from 80 to 8000 cm −1 down to temperatures as low as 10 K by applying hydrostatic pressure. In the metallic state, T > 135 K, we observe a 50% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11 kbar. In the ordered state, T < TCO, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metalinsulator transition coincide and consistently yield a linear decrease of the transition temperature TCO by 8 − 9 K/kbar. The charge disproportionation ∆ρ diminishes by 0.017 e/kbar and the optical gap ∆ between the bands decreases with pressure by -47 cm −1 /kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.

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“…Since its discovery three decades ago, the intererst in the title compound never faded because it exhibits a rich temperature-pressure phase diagram, with a number of intriguing quantum phenomena ranging from electronic ferroelectricity [15][16][17] to superconductivity, 18,19 from nonlinear transport 20,21 to zero-gap semiconductivity 22,23 characterized by Dirac cones and massless Dirac fermions, [24][25][26][27] but also the appearance of persistent photoconduction, 28 photoinduced phase transition, [29][30][31] and nonlinear ultrafast optical response.…”

Section: -5mentioning

confidence: 99%

Electrically induced phase transition inα−(BEDT-TTF)2I3: Indications for Dirac-like hot charge carriers

et al. 2016

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The two-dimensional organic conductor α-(BEDT-TTF)2I3 undergoes a metal-insulator transition at TCO = 135 K due to electronic charge ordering. We have conducted time-resolved investigations of its electronic properties in order to explore the field-and temperature-dependent dynamics. At a certain threshold field, the system switches from low-conducting to a high-conducting state, accompanied by a negative differential resistance. Our time-dependent infrared investigations indicate that close to TCO the strong electric field pushes the crystal into a metallic state with optical properties similar to the one for T > TCO. Well into the insulating state, however, at T = 80 K, the spectral response evidences a completely different electronically-induced high-conducting state. Applying a two-state model of hot electrons explains the observations by excitation of charge carriers with a high mobility. They resemble the Dirac-like charge-carriers with a linear dispersion of the electronic bands found in α-(BEDT-TTF)2I3 at high-pressure. Extensive numerical simulations quantitatively reproduce our experimental findings in all details.

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Molecular Dirac Fermion Systems — Theoretical and Experimental Approaches —

Cited by 201 publications

References 79 publications

Quantum multicriticality in disordered Weyl semimetals

Quantum multicriticality in disordered Weyl semimetals

Pressure-dependent optical investigations ofα−(BEDT-TTF)2I3: Tuning charge order and narrow gap towards a Dirac semimetal

Electrically induced phase transition inα−(BEDT-TTF)2I3: Indications for Dirac-like hot charge carriers

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