Spontaneous breaking of rotational symmetry in copper oxide superconductors

• Published in: Nature (London) Vol. 547; no. 7664; pp. 432 - 435
• Main Authors: Wu, J., Bollinger, A. T., He, X., Božović, I.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 27.07.2017; Nature Publishing Group
• Subjects: 142/126; 639/301/119/995; 639/766/119/1003; Analysis; Anisotropy; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Copper; Copper oxides; Distortion; Electric currents; Electric potential; Electron transport; electronic properties and materials; Fermi liquids; High temperature; Humanities and Social Sciences; letter; Magnetic fields; Magnetic properties; multidisciplinary; Oscillations; Oxides; Science; Single crystals; superconducting properties and materials; Superconductivity; Superconductors; Symmetry; Temperature effects; Temperature range; Thin films; Transition temperature; Voltage; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature23290

The electronic nematic phase in copper oxide superconductors is found over a broad range of temperature and doping but is not aligned with the crystal axes. A new phase of copper oxides Despite decades of study, high-temperature superconducting copper oxides can still throw up surprises. Several studies have pointed to the existence of an enigmatic anisotropic electronic state — an 'electronic nematic' phase — lurking in regions of the phase diagram. This implies that the 'normal' state of these materials, at temperatures above the superconducting transition temperature, is markedly different from that of a normal metal. Jie Wu and colleagues bring these peculiarities into sharp focus by showing that this nematic phase is widespread, as they detect it at every doping level up to room temperature, and that, even more surprisingly, it is unconnected to the crystal axes of the oxide planes in which it resides. The origin of high-temperature superconductivity in copper oxides and the nature of the ‘normal’ state above the critical temperature are widely debated 1 , 2 , 3 . In underdoped copper oxides, this normal state hosts a pseudogap and other anomalous features; and in the overdoped materials, the standard Bardeen–Cooper–Schrieffer description fails 4 , challenging the idea that the normal state is a simple Fermi liquid. To investigate these questions, we have studied the behaviour of single-crystal La 2– x Sr x CuO 4 films through which an electrical current is being passed. Here we report that a spontaneous voltage develops across the sample, transverse (orthogonal) to the electrical current. The dependence of this voltage on probe current, temperature, in-plane device orientation and doping shows that this behaviour is intrinsic, substantial, robust and present over a broad range of temperature and doping. If the current direction is rotated in-plane by an angle ϕ , the transverse voltage oscillates as sin(2 ϕ ), breaking the four-fold rotational symmetry of the crystal. The amplitude of the oscillations is strongly peaked near the critical temperature for superconductivity and decreases with increasing doping. We find that these phenomena are manifestations of unexpected in-plane anisotropy in the electronic transport. The films are very thin and epitaxially constrained to be tetragonal (that is, with four-fold symmetry), so one expects a constant resistivity and zero transverse voltage, for every ϕ . The origin of this anisotropy is purely electronic—the so-called electronic nematicity. Unusually, the nematic director is not aligned with the crystal axes, unless a substantial orthorhombic distortion is imposed. The fact that this anisotropy occurs in a material that exhibits high-temperature superconductivity may not be a coincidence.