Quantum spin liquid (QSL) is a novel state of matter which refuses the conventional spin freezing even at 0 K. Experimentally searching for the structurally perfect candidates is a big challenge in condensed matter physics. Here we report the successful synthesis of a new spin-1/2 triangular antiferromagnet YbMgGaO4 with symmetry. The compound with an ideal two-dimensional and spatial isotropic magnetic triangular-lattice has no site-mixing magnetic defects and no antisymmetric Dzyaloshinsky-Moriya (DM) interactions. No spin freezing down to 60 mK (despite θw ~ −4 K), the power-law temperature dependence of heat capacity and nonzero susceptibility at low temperatures suggest that YbMgGaO4 is a promising gapless (≤|θw|/100) QSL candidate. The residual spin entropy, which is accurately determined with a non-magnetic reference LuMgGaO4, approaches zero (<0.6%). This indicates that the possible QSL ground state (GS) of the frustrated spin system has been experimentally achieved at the lowest measurement temperatures.
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 ...
Spin-lattice coupling plays an important role in both formation and understanding of the magnetism in two-dimensional magnetic semiconductors (2DMS). In this paper, the steady pressure effects on the lattice structure, Raman resonances, and magnetization of a 2DMS Cr2Ge2Te6 have been studied by both experiments and first principles calculations. It is found that the bond length of Cr-Cr decreases, the angle of Cr-Te-Cr diverges from 90°, and the Raman modes Eg3 and Ag1 show an increase with the application of external pressure. Consequently, the magnetic phase transition temperature TC decreases from 66.6 K to 60.6 K (∼9%) as the pressure increases from 0 to 1 GPa. These pressure effects not only confirm the existence of strong spin-lattice coupling but also reveal the detailed information about the lattice deformation effect on the magnetic properties in such 2DMS, which would be a benefit for the further understanding and manipulation of the magnetism in 2D materials.
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