Diode Effect May Assist Finding Proper Superconductivity Mechanism in Copper Oxides

Abstract

We present measurements demonstrating that copper-oxide high-temperature superconductors can exhibit broken time-reversal symmetry in the absence of external magnetic fields. Using $Tl_{2}Ba_{2}CaCu_{2}O_{8}$ microbridges, we observe a pronounced superconducting diode effect at 100 K under strictly zero-field conditions. This nonreciprocal response remains unchanged in magnetic fields up to $\pm 100 Oe$. Our results are consistent with recent reports of zero-field diode behavior in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ }$ and together indicate that time-reversal symmetry breaking may be an intrinsic property of the cuprate superconducting state. These findings significantly constrain theoretical models of high-temperature superconductivity that rely on time-reversal-symmetric mechanisms.

Figures (5)

• Figure 1: Superconducting diode effect in presence of external magnetic field: the voltage down-spikes in response to applied sinusoidal current (shown in arb. units, circles). These data were obtained in our laboratory with $Nb_{3}Sn$ bridge at $2 K$; the magnetic field is applied perpendicular to the film surface, more details can be found in ref1.
• Figure 2: Appearance of the diode effect in $Tl_{2}Ba_{2}CaCu_{2}O_{8}$ superconducting bridge ($T=100 K$ and $H=0$) at the increase of the bias current. The amplitude of sinusoidal AC current is $30 mA$ in panel (a) (shown by circles, arb. units) and $35 mA$ in panel (b); the frequency is $0.2Hz$ in both cases.
• Figure 3: Superconducting transition in $Tl_{2}Ba_{2}CaCu_{2}O_{8}$ film recorded by AC susceptibility (panel (a); measurement frequency 400Hz, AC amplitude $5Oe$), and in lithographically processed bridge via resistivity (panel (b); measurement current $1\mu A$).
• Figure 4: Nonreciprocal superconducting transition in $Tl_{2}Ba_{2}CaCu_{2}O_{8}$ bridge is almost similar for $H=0$ and $H=\pm 100Oe$ (panel (a)). At the same time, as follows from panel (b), $100Oe$ exceeds the value of $H_{c1}$ of this material.
• Figure 5: Demonstration of possible nonequilibrium effects in the resistive state of the bridge. The top datasets show absolute values of voltage measured at linear current sweep to $16 mA$ and $27 mA$ with the relatively slow rate $1 mA/s$ (the lines are guide for eyes). These curves (and many others measured) coincide at intersecting current values. As soon as the current exceeds 25 mA, irreversibility appears. At the sinusoidal bias current, the level of voltage is significantly lower (the bottom data points) because of much higher scanning speed (indicated on labels), and the irreversibility comes in at noticeably higher current amplitudes.