The Griffiths singularity in a phase transition, caused by disorder effects, was predicted more than 40 years ago. Its signature, the divergence of the dynamical critical exponent, is challenging to observe experimentally. We report the experimental observation of the quantum Griffiths singularity in a two-dimensional superconducting system. We measured the transport properties of atomically thin gallium films and found that the films undergo superconductor-metal transitions with increasing magnetic field. Approaching the zero-temperature quantum critical point, we observed divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. We interpret the observed superconductor-metal quantum phase transition as the infinite-randomness critical point, where the properties of the system are controlled by rare large superconducting regions.

Interface charge doping effects on superconductivity of single-unit-cell FeSe films on SrTiO3 substrates

We investigated the dependence of T C and magnetoresistance (MR) on gate voltages ( V G ) in the 20-nm-thick high-quality La 0.6 Sr 0.4 MnO 3 (LSMO) Hall bar gated by ionic liquid (more Reversible Ferromagnetic Phase Transition in ElectrodeGated ManganitesBin Cui , Cheng Song , * Guangyue Wang , Yinuo Yan , Jingjing Peng , Jinghui Miao , Haijun Mao , Fan Li , Chao Chen , Fei Zeng , and Feng Pan * The electronic phase transition has been considered as a dominant factor in the phenomena of colossal magnetoresistance, metal-insulator transition, and exchange bias in correlated electron systems. However, the effective manipulation of the electronic phase transition has remained a challenging issue. Here, the reversible control of ferromagnetic phase transition in manganite fi lms through ionic liquid gating is reported. Under different gate voltages, the formation and annihilation of an insulating and magnetically hard phase in the magnetically soft matrix, which randomly nucleates and grows across the fi lm instead of initiating at the surface and spreading to the bottom, is directly observed. This discovery provides a conceptually novel vision for the electric-fi eld tuning of phase transition in correlated oxides. In addition to its fundamental signifi cance, the realization of a reversible metalinsulator transition in colossal magnetoresistance materials will also further the development of four-state memories, which can be manipulated by a combination of electrode gating and the application of a magnetic fi eld.

Electrical manipulation of lattice, charge, and spin has been realized respectively by the piezoelectric effect, field-effect transistor, and electric field control of ferromagnetism, bringing about dramatic promotions both in fundamental research and industrial production.

The recent observation of superconducting state at atomic scale has motivated the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we report on a superconducting phase in two-monolayer crystalline Ga films epitaxially grown on wide band-gap semiconductor GaN(0001). This phase exhibits a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings about a superconducting transition temperature Tc as high as 5.4 K, confirmed by in situ scanning tunneling spectroscopy, and ex situ electrical magneto-transport and magnetization measurements. The anisotropy of critical magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed, typical for the 2D superconductivity. Our results demonstrate a novel platform for exploring atomic-scale 2D superconductor, with great potential for understanding of the interface superconductivity.PACS numbers: 68.37. Ef, 74.55.+v, Superconductivity has recently been observed in oneatomic-layer Pb [1][2][3][4][5] and In [6,7] films grown on Si(111) substrate, at the SrTiO 3 /LaAlO 3 interface [8], and in one-unit-cell thick FeSe films on SrTiO 3 [9,10]. This has been stimulating great attention and interest for both understanding the electron pairing in quantum confined systems and also the pursuit of emergent phases of matter in the two-dimensional (2D) systems, such as the enhancement of superconducting transition temperature T c . The recent discovery of electric field induced superconductivity at SrTiO 3 surface [11] and in 2D MoS 2 crystal [12] further demonstrates the feasibility of controlling 2D superconductivity via interface engineering. Thus far, however, the nature of interface or 2D superconductivity remains obscure. Preparing more hybird heterostructures with enhanced superconductivity is particularly required but experimentally challenging.GaN, as a wide band gap and high piezo-electric constant semiconductor [13,14], is commonly used in highspeed transistors, lasers for telecommunications, and light-emitting diodes for energy efficient displays. More significantly, it has been previously shown that GaN is often wetted with one to two atomic layers of Ga atoms [15][16][17], wherein Ga is intrinsically superconductive [18][19][20]. Therfore, Ga/GaN may possibly serve an ideal system to search for enhanced superconductivity near their interface. In this work, by in situ scanning tunneling microscopy/spectroscopy (STM/STS), ex situ electrical magneto-transport and magnetization measurements, we have unambiguously demonstrated that two-monolayer (ML) Ga films (as thin as 0.552 nm) grown on GaN form a hexagonal structure and exhibit superconductivity with a T c up to 5.4 K, which differs from any previously reported stable or crystalline Ga phases [18][19][20]. The anisotropy of critical magnetic field and BerezinskiKosterlitz-Thouless (BKT)-like transition are observed, indicative of the 2D nature of superconductivity in 2 ML Ga/GaN(0001).Our STM/STS experiments are conducted in a Unisoku ultrahigh vacuum low temperature STM ...