Magnetic-field-induced superconductor-insulator-metal transition in an organic conductor: An infrared magneto-optical imaging spectroscopic study

The magnetic-field-induced superconductor-insulator-metal transition (SIMT) in partially deuterated $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Br}$, which is just on the Mott boundary, has been observed using the infrared magneto-optical imaging spectroscopy. The infrared reflectivity image on the sample surface revealed that the metallic (or superconducting) and insulating phases coexist and they have different magnetic-field dependences. One of the magnetic-field dependence is SIMT that appeared on part of the sample surface. The SIMT was concluded to originate from the balance of the inhomogeneity in the sample itself and the disorder of the ethylene end groups resulting from fast cooling.

Figure 1

(Color online) Typical reflectivity spectra $\left[R\left(\omega \right)\right]$ in the superconducting ($d[0,0]$, $T=5\mathrm{K}$, $B=0\mathrm{T}$) and antiferromagnetic insulating ($d[4,4]$, $T=5\mathrm{K}$, $B=0\mathrm{T}$) phases and their centers of spectral weight $\left(⟨\omega ⟩\right)$.

Figure 2

(Color) Magnetic-field and cooling-rate [slow cooling in (a) and fast cooling in (b)] dependencies of the spatial image of the center of the spectral weight $\left(⟨\omega ⟩\right)$ of 50% deuterated $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{C}\mathrm{N}\right)}_2\right]\mathrm{Br}$ $\left(d[2,2]\right)$ at $T=5\mathrm{K}$. The blue circles in (b) indicate the points of the different magnetic-field dependencies on the sample surface are shown in Fig. 4. The black lines indicate the rough $M\text{−}I$ boundary $\left({\omega }_{\mathit{MI}}\right)$ of $2350{\mathrm{cm}}^{-1}$ and the lower and higher wave numbers indicate the insulating and metallic (superconducting) reflectivity spectra, respectively. The hatched area is the outside of the sample. (c) Magnetic-field and cooling-rate dependencies of the normalized resistivity of $d[2,2]$ at $5.5\mathrm{K}$ for the reference. Though the cooling rate of $-0.73\mathrm{K}∕\min$ is different from that of (b), the physical character is same as shown in Ref. 5.

Figure 3

(Color online) Statistical distributions of $⟨\omega ⟩$ for $d[2,2]$ derived from Fig. 2. The Gaussian fittings to the histograms are also plotted using lines. The fitting results are discussed in the text.

Figure 4

(Color online) The magnetic-field dependencies of the $R\left(\omega \right)$ spectra (a) and of $⟨\omega ⟩$ (b) at points $\alpha$, $\beta$, $\gamma$, and $\delta$ in Fig. 2. The $R\left(\omega \right)$ spectra at points $\alpha$ and $\beta$ exhibit the magnetic-field-independent insulating $\left(I\right)$ and metallic $\left(M\right)$ characteristics, respectively. On the other hand, the spectra at points $\gamma$ and $\delta$ change from $M$ $\left(\text{or}\mathrm{SC}\right)\to I$ and $M$ $\left(\text{or}\mathrm{SC}\right)\to I\to M$, respectively, with increasing magnetic field.

Figure 5

(Color online) A conceptual $B\text{−}U∕W$ phase diagram of $d[2,2]$. The points $\alpha$, $\beta$, $\gamma$, and $\delta$ refer to the same points in Fig. 2. The solid circles and diamonds indicate the metallic $\left(M\right)$ and insulating (AFI) points, respectively, and the phase boundary is represented by a solid line. Below the upper critical field [$H_{c2}\sim 4\mathrm{T}$ (dashed line)], the superconducting state (SC) is regarded to appear in the metallic state $\left(M\right)$.