Phase Instability amid Dimensional Crossover in Artificial Oxide Crystal

Artificial crystals synthesized by atomic-scale epitaxy provide the ability to control the dimensions of the quantum phases and associated phase transitions via precise thickness modulation. In particular, the reduction in dimensionality via quantized control of atomic layers is a powerful approach to revealing hidden electronic and magnetic phases. Here, we demonstrate a dimensionality-controlled and induced metal-insulator transition (MIT) in atomically designed superlattices by synthesizing a genuine two-dimensional (2D) ${\mathrm{SrRuO}}_3$ crystal with highly suppressed charge transfer. The tendency to ferromagnetically align the spins in an ${\mathrm{SrRuO}}_3$ layer diminishes in 2D as the interlayer exchange interaction vanishes, accompanying the 2D localization of electrons. Furthermore, electronic and magnetic instabilities in the two ${\mathrm{SrRuO}}_3$ unit cell layers induce a thermally driven MIT along with a metamagnetic transition.

Figure 1

Absence of electronic reconstruction in SRO-STO SLs. (a) ${\sigma }_1(\omega )$ of ${[{(\mathrm{SRO})}_x|{(\mathrm{STO})}_y]}_{10}$ ([$x|y$]) SLs. The ratio between the thicknesses of the SRO and STO layers systematically influence ${\sigma }_1(\omega )$ of the SLs, with the appearance of an isosbesticlike point at $\sim 3.86\mathrm{eV}$. (b) The $W_s$ of the low energy region (0.04–3.86 eV, blue rhombus) increases linearly with increasing volume (thickness) the fraction of the SRO layer, whereas the $W_s$ of the high energy region (3.86–5.5 eV, red square) decreases linearly. The total $W_s$ is indicated by the black triangles. (c) Effective carrier concentration ($n_{\mathrm{eff}}$) of SLs calculated from the Drude contribution of ${\sigma }_1(\omega )$. A clear MIT is observed for the $x=1$ SL with an abrupt disappearance of the Drude peak in ${\sigma }_1(\omega )$ and the resulting $n_{\mathrm{eff}}$. The regions in red indicate the insulating phase. (d) The layered density of states (LDOS) of the [$3|8$] SL does not show any defect or interface states, indicating the absence of the electronic reconstruction. Blue (orange) colored layers are the LDOS from the STO (SRO) layers. The vertical dashed line indicates the Fermi energy level.

Figure 2

Dimensional crossover of electrical transport and magnetic properties. (a) $\rho (T)$ of the [$x|y$] SLs maintain their metallic behavior down to $x=2$. A clear MIT is observed for the $x=1$ SL, consistent with the optical results. (b) $S(T)$ of the SLs shows hole-dominant electrical transport originating from the SRO layers. The $x=1$ SL shows significantly enhanced $S(T)$ values resembling the behavior of semiconductors with bipolar conduction. (c) $M(T)$ are shown for the SLs, indicating ferromagnetic transitions at low temperature. The arrows indicate the FM $T_c$ of the SLs. The inset shows $T_c$ as a function of the SRO fraction within the SLs. Ferromagnetic (FM) ordering of the SRO vanishes for the $x=1$ SL, concomitant with the MIT. The $M(T)$ curves were measured under 100 Oe of the magnetic field along the out-of-plane direction of the thin films. (d) Upper panel: Schematic representation of the spin ordering transition of SRO across the dimensional crossover from 3D to 2D. $J_{\mathrm{intra}}$ and $J_{\mathrm{inter}}$ represent magnetic exchange coupling along and across the layers, respectively. Lower panel: DOS obtained by DFT calculations of [$x|8$] ($x=1$, 2, and 3) SLs, reproducing the experimentally observed magnetically coupled MIT. The magnetic phase transition from FMM ($x=3$) to AFMI ($x=1$) is shown, with the appearance of dimensional instability for the $x=2$ SL. (e) The calculated energy difference between AFM and FM magnetic configurations as a function of SRO atomic unit cell thickness, where the energy difference is normalized by the SRO thickness. The red (blue) region indicates the AFMI (FMM) phase.

Figure 3

Dimensional phase evolution and instability of the atomically designed SRO-STO SLs. (a)–(c) Dimensional instability in $x=2$ SL is evidenced by a thermally induced phase transition occurring simultaneously for the electronic and magnetic structures at $\sim 40\mathrm{K}$ ($T_{\mathrm{MIT}}$). (a) Although FC $M(T)$ shows a single ferromagnetic transition at $\sim 75K$, ZFC $M(T)$ shows a secondary magnetic transition. We applied 100 Oe of the magnetic field along the out-of-plane direction of the thin film. (b) $\rho (T)$ and (c) $S(T)$ indicate that this magnetic transition accompanies the transition to an insulating phase below the $T_{\mathrm{MIT}}$. (d) A temperature-magnetic field phase diagram of $x=2$ SL was plotted from $M(H)$ curves. The data points indicate the magnetic field ($H^{*}$) required for the metamagnetic transition at different temperatures. The inset shows the magnetic hysteresis behavior of $x=2$ SL at 2 K. The arrows of the inset indicate the field directions. (e) Phase diagram as a function of SRO thickness and temperature constructed from the experimental and theoretical results, highlighting the dimensionality-induced phase transition and phase instability across the dimensional crossover. The empty squares and solid triangles indicate the $T_c$ and $T^{*}=T_{\mathrm{MIT}}$, respectively. The light blue, blue, yellow, and red regions denote PMM, FMM, PMI, and AFMI phases, respectively.