Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1-unit cell (UC) limit. The ferroelectric transition temperature T(c) of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.

YbMgGaO4, a structurally perfect two-dimensional triangular lattice with an odd number of electrons per unit cell and spin-orbit entangled effective spin-1/2 local moments for the Yb(3+) ions, is likely to experimentally realize the quantum spin liquid ground state. We report the first experimental characterization of single-crystal YbMgGaO4 samples. Because of the spin-orbit entanglement, the interaction between the neighboring Yb(3+) moments depends on the bond orientations and is highly anisotropic in the spin space. We carry out thermodynamic and the electron spin resonance measurements to confirm the anisotropic nature of the spin interaction as well as to quantitatively determine the couplings. Our result is a first step towards the theoretical understanding of the possible quantum spin liquid ground state in this system and sheds new light on the search for quantum spin liquids in strong spin-orbit coupled insulators.

A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states [1][2][3][4] , most of which are characterized by exotic spin excitations with fractional quantum numbers (termed 'spinon'). Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in a triangular antiferromagnet YbMgGaO 4 . The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to a QSL state with a spinon Fermi surface in YbMgGaO 4 that has a perfect spin-1/2 triangular lattice as in the original proposal 4 of quantum spin liquids.In 1973, Anderson proposed the pioneering idea of the quantum spin liquid (QSL) in the study of the triangular lattice Heisenberg antiferromagnet 4 . This idea was revived after the discovery in 1986 of high-temperature superconductivity 5 . A QSL, as currently understood, does not fit into Landau's conventional paradigm of symmetry breaking phases 1,2,6,7 , and is arXiv:1607.02615v2 [cond-mat.str-el] 31 Jul 2017 2 instead an exotic state of matter characterized by spinon excitations and emergent gauge structures [1][2][3]6 . The search for QSLs in models and materials [8][9][10][11][12] has been partly facilitated by the Oshikawa-Hastings-Lieb-Schultz-Mattis (OHLSM) theorem that may hint at the possibility of QSLs in Mott insulators with odd electron fillings and a global U(1) spin rotational symmetry [13][14][15] .Indeed, a continuum of spin excitations has been observed in a kagome-lattice material ZnCu 3 (OD) 6 Cl 2 (refs 12,16). However, the requirement of the U(1) spin rotational symmetry, prevents the application of OHLSM theorem in strong spin-orbit-coupled (SOC) Mott insulators in which the spin rotational symmetry is completely absent. A recent theory addressed this limitation of the OHLSM theorem, arguing that, as long as time-reversal symmetry is preserved, the ground state of an SOC Mott insulator with odd electron fillings must be exotic 17 .The newly discovered triangular antiferromagnet YbMgGaO 4 (refs 18,19) displays no indication of magnetic ordering or symmetry breaking at temperatures as low as 30 mK despite approximately 4 K for the spin interaction energy scale. Because of the strong SOC of the Yb electrons, YbMgGaO 4 was the first QSL to be proposed beyond the OHLSM theorem 19 . The thirteen 4 f electrons of the Yb 3+ ion form the spin-orbit-entangled Kramers doublets that are split by the D 3d crystal electric fields [20][21][22] . At temperatures co...

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.

We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb 3+ crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO4. Three CEF excitations from the ground-state Kramers doublet are centered at the energies~! = 39, 61, and 97 meV in agreement with the e↵ective spin-1/2 g-factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between Mg 2+ and Ga 3+ giving rise to a distribution of Yb-O distances and orientations and, thus, of CEF parameters that account for the peculiar energy profile of the CEF excitations. The CEF randomness gives rise to a distribution of the e↵ective spin-1/2 g-factors and explains the unprecedented broadening of low-energy magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO4, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.PACS numbers: 75.10. Dg, 75.10.Kt, 78.70.Nx Introduction.-Quantum spin liquid (QSL) is a novel state of matter with zero entropy and without conventional symmetry breaking even at zero temperature. Such states were proposed to host 'spinons', exotic spin excitations with fractional quantum numbers [1][2][3]. Although many candidate QSL materials with two-dimensional or three-dimensional interaction topologies on the triangular, kagome, and pyrochlore lattices were reported [4][5][6][7][8][9][10][11][12][13][14][15][16][17], they typically suffer from magnetic or non-magnetic defects [18][19][20][21][22], spatial anisotropy [4,7,15], antisymmetric DzyaloshinskyMoriya anisotropy [23][24][25], and (or) interlayer magnetic couplings [25][26][27] that reduce or even completely release magnetic frustration [25,[27][28][29][30].Many of the aforementioned shortcomings can be remedied in a new triangular antiferromagnet YbMgGaO 4 that was recently reported by our group [31][32][33]. No spin freezing was detected down to at least 0.048 K, which is about 3% of the nearest-neighbor interaction J 0 ⇠ 1.5 K [33]. Residual spin entropy is nearly zero at 0.06 K, excluding any magnetic transitions at lower temperatures [31]. Below 0.4 K, thermodynamic properties evidence the putative QSL regime with temperature-independent magnetic susceptibility = const [33] and power-law behavior of the magnetic heat capacity, C m ⇠ T 2/3 [31], the observations that are consistent with theoretical predictions for the U(1) QSL ground state (GS) on the triangular lattice [34][35][36].Very recently, two inelastic neutron scattering (INS) studies of YbMgGaO 4 [37, 38] reported continuous excitations at transfer energies of 0.1 2.5 meV extending well above the energy scale of the magnetic coupling J 0 ⇠ 0.13 meV. These spectral features were identified as fractionalized excitations ('spinons') from the QSL GS [37]. Surprisingly, though, magnetic excitations remain very broad in both energy and wave-vector (Q) even in the almost fully polarized state at 7.8 T, where only narrow spin-wave e...

Muon spin relaxation (μSR) experiments on single crystals of the structurally perfect triangular antiferromagnet YbMgGaO_{4} indicate the absence of both static long-range magnetic order and spin freezing down to 0.048 K in a zero field. Below 0.4 K, the μ^{+} spin relaxation rates, which are proportional to the dynamic correlation function of the Yb^{3+} spins, exhibit temperature-independent plateaus. All these μSR results unequivocally support the formation of a gapless U(1) quantum spin liquid ground state in the triangular antiferromagnet YbMgGaO_{4}.

Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid (QSL), and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO4 stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh2 (A = alkali or monovalent ions, Re = rare earth, Ch = O, S, Se). The family compounds share the same structure (R3m) as YbMgGaO4, and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the c-axis. Specific heat and magnetic susceptibility measurements on NaYbO2, NaYbS2 and NaYbSe2 single crystals and polycrystals, reveal no structural or magnetic transition down to 50mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO4. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages. This makes the family an ideal platform for fundamental research of QSLs and its promising applications.PACS numbers: 75.10. Kt, 75.30.Et, 75.30.Gw Introduction.-The concept of quantum spin liquids (QSLs) was originally proposed by P. W. Anderson theoretically over 40 years ago [1]. It describes a highly entangled quantum state for spin degrees of freedom and was initially constructed with a superposition of spin singlets on the triangular antiferromagnet, so-called resonatingvalence-bond state [1]. Later on, the possible connection between QSLs and high-temperature superconductivity was theoretically established through doping a QSL Mott insulator [2]. Although the underlying mechanism for the high-temperature superconductivity has not yet come into a consensus, our understanding of QSLs has greatly improved, both from exactly solvable models [3,4] and several classification schemes [4,5]. On the experimental side, various frustrated magnetic materials, particularly the triangular-lattice-based antiferromagnets, were considered to be the most promising systems to realize QSLs [6]. So far, a number of compounds have been reported to host QSLs. Among them, the well-known ones include herbertsmithite and its derived compounds [7][8][9][10][11][12][13][14], and triangular organics [15][16][17][18][19]. The magnetic ions in most of these compounds are 3d transition metal ions Cu 2+ with S = 1/2, which may be crucial to enhance quantum fluctuations.Quite recently, frustrated materials with magnetic rare-earth ions are proposed to be promising QSL candidates [20]. These include the well-known pyrochlore ice materials [21][22][23][24][25][26][27][28][29][30], the kagome magnet [31,32], and the triangular lattice magnets [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. The local degree

No abstract