Conductivity of the interface for spin-polarized electrons in the pseudogap state of underdoped $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$

We report resistance-temperature, conductance-voltage, and voltage–magnetic-field measurements in a range of small-area junctions between colossal magnetoresistive materials and the high-temperature superconductor $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ (YBCO). We identify two forms of behavior. In one form the resistance of the junction falls with decreasing temperature and is associated with zero bias conductance peaks. In the other form the resistance rises below some characteristic temperature $T^{*}$. In this latter case there is no zero bias conductance peak, but there are small peaks in the voltage–magnetic-field behavior below $T^{*}$. We tentatively associate $T^{*}$ with the pseudogap temperature and argue that the observed effects result from an interaction between the spin-polarized current and the antiferromagnetic spin fluctuations of the pseudogap state.

Figure 1

(a) Side view of specimen. 1: Au, 2: LCMO, 3: STO, 4: YBCO, and 5: STO substrate. (b) Plan view of specimen showing lead connection for measurement of junction resistance. (c) Measurement of resistance of underlying YBCO film. Typical film thicknesses are YBCO $100–200\mathrm{nm}$, STO $10–30\mathrm{nm}$, and LCMO $100–200\mathrm{nm}$.

Figure 2

Cross-geometry of LCMO-YBCO contacts.

Figure 3

Resistance vs temperature characteristics $R_j\left(T\right)$ of six junctions and of the underlying YBCO film $R_{\mathrm{YBCO}}\left(T\right)$ in sample 1.

Figure 4

Conductance vs voltage curves for two LCMO-YBCO junctions of type A at temperatures in order of the decrease of the amplitude: (a) 8.8, 19.1, 32.9, 41.6, 46.3, 48.9, 51.4, 54.3, 57.1, 59.5, 60.8, 61.6, 63.0, 65.2, 66.8, 68.9, and $97.7\mathrm{K}$; and (b) 18.7, 32.6, 41.7, 44.6, 59.4, 64.4, 68.6, 73.1, and $96.9\mathrm{K}$. The broad shoulder disappears at around the $T_c$ of the YBCO $\sim 62\mathrm{K}$.

Figure 5

(a) Conductance-voltage curves for a LNO-YBCO sample at temperatures (up to down) 18.8, 32.8, 41.8, 50.7, 59.6, 68.9, 78.2, and $86.9\mathrm{K}$, and (b) junction resistance vs temperature for measuring current $8\mathrm{mA}$ (1) and $70\mu \mathrm{A}$ (2). Note the much lower $R_j$ than in the LCMO-YBCO sample 1.

Figure 6

(a) Two consecutive conductance∕voltage curves at $18.8\mathrm{K}$ and (b) junction 1 and YBCO 2 resistance vs temperature for a gold-YBCO sample. Note the much higher $R$ of the junction than in the LCMO∕YBCO sample 1.

Figure 7

Normalized voltage (or normalized resistance) vs field at $3\mathrm{mA}$ for a junction of type B in the YBCO-LSMO sample 5, at two temperatures: $78.3\mathrm{K}$, which is below $T^{*}$, and $274.0\mathrm{K}$, which is above $T^{*}$. The magnetization curve at $80.0\mathrm{K}$ for the same sample is also shown.

Figure 8

Amplitude of peaks in $R_j\left(H\right)$ vs temperature for an LSMO-YBCO junction of type B.

Figure 9

$R_j\left(T\right)$ and $R_{\mathrm{YBCO}}\left(T\right)$ for the LSMO-YBCO sample 5.

Figure 10

Temperature dependence of the amplitude of the coercive-field anomaly in the crosslike LCMO-YBCO contact.