High bias anomaly in YBa${}_2$Cu${}_3$O${}_{7-x}$/LaMnO${}_{3+\delta }$/YBa${}_2$Cu${}_3$O${}_{7-x}$ superconductor/ferromagnetic insulator/superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa${}_2$Cu${}_3$O${}_{7-x}$/LaMnO${}_{3+\delta }$/YBa${}_2$Cu${}_3$O${}_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a steplike onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a different type of the superconducting proximity effect of nonequilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly nonequilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.

Figure 1

(a) Magnetization, (b) resistance, and (c) optical spectra of a single 100 nm thick LMO film showing its ferromagnetic insulator properties. (d) Field-cooled magnetization of a YBCO (10 nm)/LMO (10 nm)/YBCO (10 nm) trilayer at 5 kOe and a $\left[\text{YBCO}\right(10\text{nm})/\text{LMO}(10\text{nm}\left)\right]\times 10$ superlattice at 1 kOe. (e) Lateral resistance of the YBCO (10 nm)/LMO (10 nm)/YBCO (10 nm) trilayer with the contacts on the top YBCO layer. (f) Optical conductivity of the $\left[\text{YBCO}\right(10\text{nm})/\text{LMO}(10\text{nm}\left)\right]\times 10$ superlattice.

Figure 2

(a) Top panel: Scanning electron microscopy (SEM) image of a 3D-FIB patterned junction No. 2. Bottom panel: Side-view sketch of the structure (not in scale). Arrows indicate the current flow path. (b) In-plane resistive transition of the same bridge before the final FIB cut. A sharp superconducting transition occurs at $T_c\simeq 86$ K. (c) Temperature dependence of the out-of-plane zero-bias resistance $R_0\left(T\right)$ (in the semilogarithmic scale) of junction No. 2 at $H=0$. The transition to the insulating behavior occurs at $T<T_{\mathrm{Curie}}\simeq 150$ K. Note that $R_0\left(T\right)$ reveals no sign of superconductivity at $T<T_c$, indicating the lack of a proximity effect at low bias. (d)–(f) Magnetic field dependence of the $I$-$V$ curves of junction No. 4: (d) in the paramagnetic state $T=200$ K, (f) close to the Curie temperature $T=150\text{K}\simeq T_{\mathrm{Curie}}$, and (f) in the FI state $T=100\text{K}<T_{\mathrm{Curie}}$. Negative colossal magnetoresistance appears in the FI state.

Figure 3

(a) Bias dependence of the magnetoresistance (MR) at in-plane fields of $H=1$ and 14 T for junction No. 4. The MR reaches $-600\%$ at $H=14$ T. (b) The same as in (a), normalized by the magnetic field. It is seen that at low bias $\left|V\right|<0.1$ V the MR is approximately linear in field up to 14 T, at intermediate bias the MR is nonlinear in $H$, and it again becomes approximately linear at a higher bias $\left|V\right|\gtrsim 1.5$ V. (c) Nonlinear field dependence of the junction resistance at an intermediate bias $V=0.7$ V at $T=2$ (left axis) and 100 K (right axis).

Figure 4

(a) Large bias dc resistance of junction No. 4 at $H=0$ and 10 T applied along the $ab$ plane and the $c$-axis direction. The upturn at $T<T_{\mathrm{Curie}}\sim 150$ K marks the transition to the FI state of LMO while the downturn at $T_c\simeq 86$ K indicates the appearance of a superconducting proximity effect. (b) $I$-$V$ curves of junction No. 4 at different $T$ and at $H=0$. Below $T_c$, a steplike onset of excess current appears at a large bias voltage of about 1 V. (c) Magnetic field dependence of the $I$-$V$ curves at $T=30$ K. The steplike increase of the current is rapidly suppressed by a magnetic field. (d) $T$ dependence of the excess current (step height) $\Delta I$ at high bias. (e) and (f) Differential conductance $dI/dV$ of junction No. 4 (e) at different temperatures and $H=0$, and (f) at different $H$ and $T\simeq 2$ K.

Figure 5

(a) Sketch of the tunneling diagram of our S/FI/S junctions. Horizontal arrows indicate Fowler-Nordheim type tunneling of electrons into the conduction band (minority spin $\downarrow$) and holes into the valence band (majority spin $\uparrow$) of LMO at $eV>U+\Delta$. Inset: $I$-$V$ curve at $T=1.9$ K showing that the current is blocked in regime A at $V\lesssim 0.3$ V. (b) Fowler-Nordheim plots for junction No. 4 at $T=1.9$ K and different $H$. (c) Modified tunneling diagram with the proximity effect in the conduction and valence bands of LMO. The proximity effect leads to a reduction of the electric field in the central part of the LMO barrier and concentration near the interfaces. The shrinkage of the effective barrier width gives rise to an excess current.

Figure 6

Radiation detection from junction No. 4 at $T=1.8$ K and $H=0$. (a) The $I$-$V$ of the generator junction No. 4. (b) The signal of the detector junction. The appearance of a significant photon emission above the polaronic mobility edge in LMO (dashed line) and resonant enhancement of emission at the step (solid line) are seen.