The layered compound α-RuCl3 is composed of a honeycomb lattice of magnetic Ru 3+ ions with the 4d 5 electronic state. We have investigated the magnetic properties of α-RuCl3 via magnetization and specific heat measurements using single crystals. It was observed that α-RuCl3 undergoes a structural phase transition at Tt ≃ 150 K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related CrCl3. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic g-factors, i.e., g ab ≃ 2.5 and gc ≃ 0.4 for magnetic fields parallel and perpendicular to the ab plane, respectively. These g-factors and the obtained entropy indicate that the effective spin of Ru 3+ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for H ab is obtained. We discuss the strongly anisotropic g-factors in α-RuCl3 and deduce that the exchange interaction is strongly XY-like. α-RuCl3 is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.

We report the results of magnetization and specific heat measurements on Ba3CoSb2O9, in which the magnetic Co 2+ ion has a fictitious spin-1 2, and show evidence that a spin-Heisenberg antiferromagnet on a regular triangular lattice is actually realized in Ba3CoSb2O9. We found that the entire magnetization curve including the one-third quantum magnetization plateau is in excellent agreement with theoretical calculations at a quantitative level.Exploring the ground state of a frustrated quantum magnet has been one of the main subjects of condensed matter physics 1-3 . A long theoretical debate reached a consensus that a two-dimensional (2D) spin-1 2 TLHAF has an ordered ground state of the 120• spin structure with an extremely small sublattice magnetization 4-7 . Although the zero-field ground state is qualitatively the same as that for the classical spin, the ground state in a magnetic field H cannot be determined uniquely only from the classical model. The classical equilibrium condition is given by S 1 + S 2 + S 3 = gµ B H/(3J) with the sublattice spin Si. Because there are an infinite number of spin states that satisfy this condition, the classical ground state is continuously degenerate. The ground state of a small spin TLHAF in a magnetic field is essentially determined by the quantum fluctuation energy.A remarkable quantum effect is that an up-up-down spin state, which appears in a magnetic field for the classical model, can be stabilized in a finite magnetic field range, so that the magnetization curve has a plateau at onethird of the saturation magnetization 8-14 .The nature of the quantum mechanical ground state in a magnetic field is strongly reflected in the magnetization process. The magnetization process for a 2D spin-1 2 TLHAF, which exhibits the most pronounced quantum effect, was calculated energetically by means of spin wave theory 9,11 , coupled cluster method 12 and exact diagonalization 13,14 . The calculated magnetization curves are greatly different from that for the classical spin. However, experimental verification of the theoretical results has not been conducted at a quantitative level.Experimentally, Cs 2 CuCl 4 15 , Cs 2 CuBr 4 16,17 and κ-(BEDT-TTF) 2 Cu 2 (CN) 3 18 have been actively investigated as spin-1 2 TLHAFs. However, the triangular lattice in these substances is not regular but distorted, and thus, the exchange interaction is spatially anisotropic. Cs 2 CuCl 4 and Cs 2 CuBr 4 also exhibit a large antisymmetric interaction of the Dzyaloshinsky-Moriya (DM) type. Although the quantum magnetization plateau has been actually observed in Cs 2 CuBr 4 16,17 , the magnetization process is anisotropic and the magnetization plateau is not observed for a magnetic field perpendicular to the triangular lattice plane. In this letter, we present the results of magnetization and specific heat measurements on Ba 3 CoSb 2 O 9 and provide evidence that Ba 3 CoSb 2 O 9 closely approximates to the ideal spin- This substance crystallizes in a highly symmetric hexagonal structure, P 6 3 /mmc, whi...

We have performed high-field magnetization and ESR measurements on Ba3CoSb2O9 single crystals, which approximates the two-dimensional (2D) S = 1/2 triangular-lattice Heisenberg antiferromagnet. For an applied magnetic field H parallel to the ab-plane, the entire magnetization curve including the plateau at one-third of the saturation magnetization (Ms) is in excellent agreement with the results of theoretical calculations except a small step anomaly near (3/5)Ms, indicative of a theoretically undiscovered quantum phase transition. However, for H c, the magnetization curve exhibits a cusp near Ms/3 owing to the weak easy-plane anisotropy and the 2D quantum fluctuation. From a detailed analysis of the collective ESR modes observed in the ordered state, combined with the magnetization process, we have determined all the magnetic parameters including the interlayer and anisotropic exchange interactions.PACS numbers: 75.10. Jm, 75.45.+j, 75.60.Ej, Over the past decades, there has been considerable interest in frustrated quantum magnets, owing to a rich variety of exotic quantum phenomena [1][2][3]. For classical spins with an antiferromagnetic coupling, a geometric frustration suppresses the long-range ordering, leading to a degenerate ground state. The degeneracy can be destroyed by quantum fluctuations, which emerge not only through an interplay of strong geometric frustration, low dimensionality, and small spin, but also through the application of a magnetic field. Despite intensive research efforts, the detailed mechanism of the quantum effects, e.g., the ground state property [4,5], has still been highly controversial.One macroscopic manifestation of the quantum phenomena is the stabilization of the "up-up-down" spin structure under a magnetic field, predicted for a twodimensional (2D) triangular-lattice Heisenberg antiferromagnet (TLHAF) with a small spin [6,7]. In a magnetization process, the nonclassical anomaly appears as a plateau in a finite field range at one-third of the saturation magnetization M s , hereafter referred to as the M s /3 plateau. In a classical picture, a monotonic increase in the magnetization is expected up to M s . A number of theoretical approaches for explaining the quantum mechanism of the M s /3 plateau have been proposed [8][9][10][11][12][13][14]. Thus far, however, few numbers of definite experimental results reserved judgment on the issue. This is mainly due to the experimental difficulty in growing the model material, let alone in observing the M s /3 plateau purely driven by quantum fluctuations. In fact, most of the TLHAFs ever studied, such as Cs 2 CuBr 4 , [15,16] have a distorted triangular lattice, which induces an antisymmetric Dzyaloshinsky-Moriya (DM) interaction.It is believed that the spin state in the lower-field range above the higher edge field of the M s /3 plateau is the 2 : 1 canted coplanar state that is a continuous variant of the up-up-down state [7][8][9][10][11]. However, whether the 2 : 1 canted coplanar state is stable up to the saturation or a new quan...

We report the direct observation of dioxygen molecules physisorbed in the nanochannels of a microporous copper coordination polymer by the MEM (maximum entropy method)/Rietveld method, using in situ high-resolution synchrotron x-ray powder diffraction measurements. The obtained MEM electron density revealed that van der Waals dimers of physisorbed O2 locate in the middle of nanochannels and form a one-dimensional ladder structure aligned to the host channel structure. The observed O-O stretching Raman band and magnetic susceptibilities are characteristic of the confined O2 molecules in one-dimensional nanochannels of CPL-1 (coordination polymer 1 with pillared layer structure).

High-field magnetization measurements up to 57 T have been performed at 0.08 K in a single crystal of a two-dimensional spin-gap material SrCu 2 (BO 3 ) 2 . We successfully observed the predicted plateau at 1/3 of the total magnetization around 50 T, in which the magnetic superstructure is characterized by a novel stripe order of triplets. The 1/3 plateau is much wider than the previously observed 1/4 and 1/8 plateaux.

Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level (E F ) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to E F by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-T c topological superconductivity.High-T c iron-based superconductors feature multiple bands near E F , which enhances the difficulty in understanding the details of unconventional pairing 1-3 . It, however, also allows for a wealth of, possibly topologically non-trivial, electronic bands, of which a recent example is the TI states discovered in the ironbased superconductor Fe(Te,Se) 4 , hinting at a promising direction to realize topological superconductivity and MBSs 5-9 . In view of Fe(Te,Se), a pressing subsequent question is to which extent this marks a generic phe-nomenon in different classes of iron-based high-T c superconductors. In this work, we find that the emergence of non-trivial topological bands near the Fermi level is indeed a common feature of various iron-based superconductors. Our first-principles calculations reveal that BaFe 2 As 2 , LiFeAs and Fe(Te,Se) all exhibit band inversions along k z . To confirm these calculations, the band structures of Li(Fe,Co)As and Fe(Te,Se) were investigated by laser-based high-resolution ARPES. Firstly, we observe that TI bands reminiscent of Fe(Te,Se) exist in Li(Fe,Co)As as well, supporting the generic existence of non-trivial topology in iron-based superconductors. Secondly and more interestingly, we predict and observe TDS bands in Li(Fe,Co)As and Fe(Te,Se), which we investigate via high-resolution ARPES, spin-resolved ARPES (SARPES), and magnetoresistance (MR) measurements. Finally, we discuss the phase diagram of these topological high-T c compounds as a function of Fermi level (doping). The combination of topological states and superconductivity may produce not only surface topological superconductivity deriving from the TI edge states, but also bulk topological superconductivity from the TDS bands.Normal insulator (NI), TI, and TDS constitute topologically disti...

We studied the magnetic properties in a non-magnetic heavy-fermion compound CeIrIn 5 and an antiferromagnetic compound CeRhIn 5 with the tetragonal structure. High-field magnetization of CeIrIn 5 shows a weak metamagnetic transition around 420 kOe for the field along the [001] direction, while the magnetization gradually increases up to 500 kOe for [110]. On the other hand, we observed a two-step metamagnetic transition for the field perpendicular to [001] at 20 and 500 kOe in CeRhIn 5 . The magnetization is of the x-y type. The former transition indicates a 1/ cos θ dependence, where the θ means a tilted angle of the field direction from the (001) plane. From the magnetic susceptibility and high-field magnetization measurements, we formed a magnetic phase diagram in CeRhIn 5 . The temperature dependences of the magnetic susceptibility and thermal expansion coefficient of both compounds were analyzed on the basis of the crystalline electric field model.

We have synthesized high-quality single crystals of volborthite, a seemingly distorted kagome antiferromagnet, and carried out high-field magnetization measurements up to 74 T and ^{51}V NMR measurements up to 30 T. An extremely wide 1/3 magnetization plateau appears above 28 T and continues over 74 T at 1.4 K, which has not been observed in previous studies using polycrystalline samples. NMR spectra reveal an incommensurate order (most likely a spin-density wave order) below 22 T and a simple spin structure in the plateau phase. Moreover, a novel intermediate phase is found between 23 and 26 T, where the magnetization varies linearly with magnetic field and the NMR spectra indicate an inhomogeneous distribution of the internal magnetic field. This sequence of phases in volborthite bears a striking similarity to those of frustrated spin chains with a ferromagnetic nearest-neighbor coupling J_{1} competing with an antiferromagnetic next-nearest-neighbor coupling J_{2}.