Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Phys Rev Lett. 2013 Aug 9;111(6):067003. doi: 10.1103/PhysRevLett.111.067003. Epub 2013 Aug 8.
We investigate the field-tuned quantum phase transition in a 2D low-disorder amorphous InO(x) film in the frequency range of 0.05 to 16 GHz employing microwave spectroscopy. In the zero-temperature limit, the ac data are consistent with a scenario where this transition is from a superconductor to a metal instead of a direct transition to an insulator. The intervening metallic phase is unusual with a small but finite resistance that is much smaller than the normal state sheet resistance at the lowest measured temperatures. Moreover, it exhibits a superconducting response on short length and time scales while global superconductivity is destroyed. We present evidence that the true quantum critical point of this 2D superconductor metal transition is located at a field B(sm) far below the conventionally defined critical field B(cross) where different isotherms of magnetoresistance cross each other. The superfluid stiffness in the low-frequency limit and the superconducting fluctuation frequency from opposite sides of the transition both vanish at B≈B(sm). The lack of evidence for finite-frequency superfluid stiffness surviving B(cross) signifies that B(cross) is a crossover above which superconducting fluctuations make a vanishing contribution to dc and ac measurements.
我们在 0.05 到 16GHz 的频率范围内，利用微波光谱学研究了二维低无序非晶 InO(x) 薄膜中的场调量子相变。在零温极限下，交流数据与超导态到金属态的转变情景一致，而不是直接转变为绝缘体。这一中间金属相不寻常，电阻很小但不为零，在测量的最低温度下远小于正常态的薄层电阻。此外，它在全局超导性被破坏的情况下，在短长度和时间尺度上表现出超导响应。我们提供的证据表明，这种二维超导-金属相变的真实量子临界点位于常规定义的临界磁场 B(cross) 以下的远场 B(sm)，在那里，磁阻的不同等温热线相互交叉。在低频极限下的超流劲度和在相变两侧的超导涨落频率都在 B≈B(sm) 处消失。在 B(cross) 处没有证据表明有限频率超流劲度存在，这表明 B(cross) 是一个交叉点，超过该点后，超导涨落对直流和交流测量的贡献可以忽略不计。
