Fermi surface reconstruction in the magnetic-field-induced superconductor $\kappa \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Br}}_4$

Shubnikov-de Haas (SdH) and angular dependent magnetoresistance oscillations (AMROs) are measured in the organic superconductor with magnetic anions, $\kappa \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Br}}_4$. In the paramagnetic phase $({\mu }_{0}H>{\mu }_0{H}_c=5.5\mathrm{T})$, the AMROs and the SdH oscillations are clearly observed, which are basically consistent with the band structure calculation. In the canted antiferromagnetic phase $(H<H_c)$, a low frequency SdH oscillation is observed. The observation of the oscillation is not interpreted by the simple FS reconstruction due to the long range order of the Fe $3d$ spins and implies that the strong correlation between the $\pi$ and $d$ electrons modifies the reconstruction. No sign of the enhancement of the critical quantum fluctuation at $H_c$ is observed in the SdH oscillations, the reason of which is also discussed.

Figure 1

The interlayer resistance as a function of magnetic field perpendicular to the conducting ac layers at $0.3\mathrm{K}$. Insets show the semilog plots of the FT spectra in low and high fields.

Figure 2

(a) FS in the PM phase. (b) Reconstructed FS in the CAF phase. The thick square shows the FBZ in each phase.

Figure 3

(a) FT spectra of $\delta$ at various temperatures. (b) Temperature dependence of the FT amplitude divided by temperature (mass plot).

Figure 4

SdH oscillations at various field directions. Inset shows the ${\cos }^{-1}\theta$ dependence of the frequencies.

Figure 5

Field dependence of the AMROs for $\varphi =90°$ at $1.8\mathrm{K}$. The lower part shows the data in low fields.

Figure 6

The second derivative of the resistance as a function of $\tan \theta$ for $\varphi =-40°$. Two periods denoted by $\Delta$ and ${\Delta }^{\prime }$ are evident.

Figure 7

AMROs for various azimuthal angles $\varphi$ at $13.8\mathrm{T}$ and $1.8\mathrm{K}$.

Figure 8

Polar plot of $k_{\Vert }^{\max }$ for $\Delta$ (circles) and ${\Delta }^{\prime }$ (triangles). The broken lines show the fitted curves. The hollow triangles are eliminated from the fitting procedure because they are caused by the Lebed resonance. The FS’s obtained as the best fit are shown by the solid lines.

Figure 9

AMROs for $\varphi =-20°$, where the contribution from the Q1D FS is evident. The second derivative of the data as a function of $\tan \theta$ is shown in the lower inset. Upper inset shows the $\varphi$ dependence of the period $\chi$ in $\tan \theta$. The broken lines show the fitted curves.

Figure 10

AMROs near $H\Vert a$ at various magnetic fields up to $25\mathrm{T}$.

Figure 11

(a) Oscillatory parts of $\alpha$ and $\delta$ at $30\mathrm{mK}$. (b) Field dependence of the oscillation frequencies. (c) Field dependence of the effective masses. Neither distinct field dependence nor enhancement is observed in the frequencies and the effective masses.