Anomalous metallic state regulated by magnetic field at LaAlO₃/SrTiO₃ heterointerface

Abstract

<sec>Since the discovery of two-dimensional electron gas with high mobility at the LaAlO₃/SrTiO₃ heterointerface, many physical properties such as two-dimensional superconductivity, magnetism and spin-orbit coupling have been widely studied. The origin of the transition from quantum superconductor to metal at zero temperature in two-dimensional superconductor is still an open problem, which has been discussed intensely. According to the conventional theory, when the temperature is close to zero, the superconductor-insulator transition can be observed by applying a magnetic field or magnetic field effect of disorder, and the ground state should be superconducting or insulating.</sec><sec>However, when Jaeger et al. (Jaeger H M, Haviland D B, Orr B G, Goldman A M <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevB.40.182">1989 <i>Phys. Rev. B</i> <b>40</b> 182</ext-link>) studied the relationship between superconductivity evolution and thickness and temperature in a superconducting granular film, they found that there exists an intermediate metal state that can destroy the direct transition between superconducting and insulating. The intermediate metal state is characterized by the existence of saturation resistance at superconducting transition temperature, and the corresponding ground state is called anomalous metallic state. In addition to the saturation of resistance at low temperature, the characteristics of an anomalous metallic state also include the linear current-voltage (<i>I-V</i>) characteristics in the low current range, the giant positive magnetoresistance (MR), the vanishing of Hall resistance (<i>R</i>_<i>xy</i>), and the tuning capability adjusted by changing a variety of parameters including degree of disorder, gate voltage and magnetic field.</sec><sec>In this work, we systematically investigate the electrical transport properties of LaAlO₃/SrTiO₃ (001) heterointerface in a perpendicular magnetic field at low temperature. The <i>R-T</i> curves and the <i>I-V</i> characteristics in zero magnetic field show that LaAlO₃/SrTiO₃ (001) heterointerface is in a superconducting state. However, after a small magnetic field is applied, the LaAlO₃/SrTiO₃ (001) heterointerface has the characteristics of resistance saturation at low temperature, linear <i>I-V</i> characteristics, giant positive MR, abnormal Hall response, indicating the clear characteristics of an anomalous metallic state. The sample undergoes a transition from quantum superconductor to metal at temperatures approaching to zero.</sec><sec>In addition, we observe that the anomalous metallic state in an unusually large region under the action of magnetic field, and our main observations are summarized in the <i>H-T</i> phase diagram. By analyzing the relationship between the resistance of the anomalous metallic regime and the magnetic field, and the vanish of Hall resistance, we infer that the anomalous metallic state observed in LaAlO₃/SrTiO₃ (001) heterointerface can be explained by Bose metal model. According to our findings, the magnetic field regulated LaAlO₃/SrTiO₃ (001) heterointerface appears as a special platform to study the details of anomalous metallic state in a controllable way.</sec>