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The energies as a function of the magnetic field ($H$) and the pressure arestudied theoretically in the tight-binding model for the two-dimensionalorganic conductor,  $\alpha$-(BEDT-TTF)$_2$I$_3$, in which massless Dirac fermions are realized.The effects of the uniaxial pressure ($P$) are studied by using thepressure-dependent hopping parameters. The system is semi-metallic with thesame area of an electron pocket and a hole pocket at $P < 3.0$~kbar, where theenergies $(\varepsilon_{\rm D}^0$) at the Dirac points locate below the Fermienergy $(\varepsilon_{\rm F}^0$) when $H=0$. We find that at $P=2.3$~kbar theDirac cones are critically tilted. In that case a new type of band crossingoccurs at "three-quarter"-Dirac points, i.e., the dispersion is quadratic inone direction and linear in the other three directions. We obtain newmagnetic-field-dependences of the Landau levels $(\varepsilon_n)$;$\varepsilon_n-\varepsilon^0_{\textrm{D}} \propto (n H)^{4/5}$ at $P=2.3$~kbar("three-quarter"-Dirac points) and $|\varepsilon_n-\varepsilon_{\rm F}^0|\propto (n H)^2$ at $P=3.0$~kbar (the critical pressure for the semi-metallicstate). We also study the magnetization as a function of the inverse magneticfield. We obtain two types of quantum oscillations. One is the usual de Haasvan Alphen (dHvA) oscillation, and the other is the unusual dHvA-likeoscillation which is seen even in the system without the Fermi surface.

Three-quarter Dirac points, Landau levels and magnetization in $\alpha$-(BEDT-TTF)$_2$I$_3$