Critical behavior of quasi two-dimensional organic-inorganic halide perovskites (C6H5CH2CH2NH3)(2)CuCl4 is investigated using magneto-thermal and isothermal magnetic properties along the easy axis. Banerjee's criterion indicates that phase transition from paramagnetic to ferromagnetic phases below TC = 9.4 K is of second-order. Scaling analysis reveals that critical exponent beta from spontaneous magnetization in the critical region below TC and delta from critical isotherm at TC are found to be 0.22(3) and 9.28-9.4, close to 2D finite XY model. Meanwhile, gamma from magnetic susceptibility in the critical region above TC is gradually decreased from 2.4 at high-temperature region. Critical exponents from magneto-thermal properties are found to be consistent with those determined from magnetization isotherms. The reliability of critical values is verified using the scaling hypothesis. Our results evidence that (C6H5CH2CH2NH3)(2)CuCl4 crossovers from isotropic 2D Heisenberg to anisotropic 3D models towards TC and can be promising candidates for magnetocaloric materials for hydrogen reliquefaction.

By performing resonant inelastic x-ray scattering (RIXS) measurements at the Ir $\mathit{L_{\mathrm{3}}}$ edge, we have investigated the low-energy elementary excitations in a series of double perovskite iridate single crystals, La$_{2}$$\mathit{M}$IrO$_{6}$ ($\mathit{M}$ = Co, Ni, and Zn). Almost dispersionless magnetic excitations at $\sim$ 42(6) meV and $\sim$ 35(5) meV have been observed in crystals containing magnetic 3$\mathit{d}$ ions, La$_{2}$CoIrO$_{6}$ and La$_{2}$NiIrO$_{6}$, respectively. In contrast, this low-energy magnetic excitation is absent in La$_{2}$ZnIrO$_{6}$ in which the 3$\mathit{d}$ ions are non-magnetic, suggesting the importance of 3$\mathit{d}$-5$\mathit{d}$ hybridization in the magnetic properties of these double perovskite iridates. The magnetic excitation is suppressed completely above the magnetic ordering temperature, suggesting the inadequacy of using a simple spin Hamiltonian to describe magnetism of these materials.

The transition metal dichalcogenide TiSe2 is an ideal correlated system for studying the interplay between superconductivity (SC) and a charge density wave (CDW) because both symmetry-breaking phases can be easily controlled by either Cu intercalation or physical pressure. SC appears in proximity to a CDW quantum critical point (QCP) induced by both Cu intercalation and applied pressure, raising the possibility of CDW-driven SC. Here, we report tuning the CDW QCP by simultaneously controlling Cu intercalation and external pressure and the appearance of a SC dome centered on the tunable QCP. When subjected to pressure, CDW ordering of Cuintercalated Cu0.025TiSe2 is completely suppressed at 2.3 GPa, where the residual resistivity and the resistivitytemperature exponent decrease sharply, indicating the presence of the CDW QCP. The upper critical field of Cu0.025TiSe2 is 3.51 kOe, 16 times larger than that of pristine TiSe2, and its temperature dependence is linear, indicating that SC of TiSe2 is switched from the two-dimensional- to anisotropic three-dimensional-like by Cu intercalation. These discoveries show that the simultaneous application of Cu intercalation and pressure move the CDW QCP and that the highest SC transition temperature is pinned to the QCP, suggesting that the SC in TiSe2 is strongly correlated with CDW quantum criticality.

Herein, we report the magnetodielectric effect in a single crystal of (CH3NH3)2MnCl4, an organic–inorganic hybrid layered perovskite having a composition of A2MX4 (A = organic anion, M = metal divalent ion, X = halogen cation). Magnetic property measurements reveal that a magnetic phase transition to antiferromagnetism appears at 47 K and a spin-flop transition occurs at 3.5 T in the magnetization (M) curve. We observed a considerable maximum magnetodielectric (MD) effect of 0.4% at 8 K. Furthermore, it was found that the MD effect is proportional to M2 in low magnetic fields and the MD effect starts to saturate at the spin-flop transition field. We proposed a magnetic structure model to explain the mechanism of the peculiar step-like MD effect induced by the spin reorientation.

Understanding characteristic energy scales is a fundamentally important issue in the study of strongly correlated systems. In multiband systems, an energy scale is affected not only by the effective Coulomb interaction but also by the Hund’s coupling. Direct observation of such energy scale has been elusive so far in spite of extensive studies. Here, we report the observation of a kink structure in the low energy dispersion of NiS 2− x Se x and its characteristic evolution with x , by using angle resolved photoemission spectroscopy. Dynamical mean field theory calculation combined with density functional theory confirms that this kink originates from Hund’s coupling. We find that the abrupt deviation from the Fermi liquid behavior in the electron self-energy results in the kink feature at low energy scale and that the kink is directly related to the coherence-incoherence crossover temperature scale. Our results mark the direct observation of the evolution of the characteristic temperature scale via kink features in the spectral function, which is the hallmark of Hund’s physics in the multiorbital system.

Metal Organic Frameworks (MOFs) are a group of multiferroic candidates that exhibit various optical, electrical, magnetic, and structural properties depending on the type of organic cations, transition metals and halogen elements, and their molar ratio. (C 6 H 5 CH 2 NH 3 ) 2 [CuCl 4 ] (BZACC), one of the MOF materials, has a special hybrid layered structure of organic and inorganic layers. Magnetic property measurement showed that BZACC exhibits weak in-plane ferromagnetism below the Curie point ( T C ) of 10.25 K and the saturated magnetic moment of 1 µ B per formula unit at 2 K. When the magnetic field is less than 0.1 T along the c axis, it exhibits weak antiferromagnetism below 8 K. From the measurement of the dielectric constant with external magnetic field, we observed a maximum magnetodielectric (MD) effect of 0.4% at 2 K. Furthermore, it was found that the magnetic-field dependence of the MD effect is similar to that of the second derivative of magnetization with respect to magnetic field. We suggest that the observed MD effect is the consequence of the magnetic-field-induced spin structure change in the inorganic layer, which induces the change in the interlayer magnetic coupling and, as a result, induces the change in the dielectric constant by the applied magnetic field.

We have measured the magneto-optic (MO) properties of film-type bismuth substituted yttrium iron garnets (Bi1.5:YIG, Bi1.5Y1.5Fe5O12) prepared by using metalorganic-decomposition (MOD) method on glass substrates at the 1310-nm and 1550-nm wavelengths. The Verdet constant of the Bil.5:YIG film in the unsaturated linear magnetization region has been experimentally determined from a sensitive measurement of the Faraday rotation of the Bi1.5:YIG films with a lock-in amplifier and an auto-balanced photoreceiver under alternating magnetic fields. The Bi:YIG films have been deposited on silica glass substrates without any buffer layer and with one of buffer layers of Bi1Y2Fe5O12 (Bi1:YIG) and Bi1Fe4Ga1Nd2O12 (Bi1:NIGG) which are used to compensate mismatch of the lattice constant and thermal expansion coefficient between the film and substrate. The maximum value of the measured Faraday rotation of the Bi1.5:YIG film was over 94.6 and 156.5 degrees/cm for an applied unsaturated magnetic field of 100 Gauss at wavelengths of 1310 and 1550 nm, respectively, when it was prepared at annealing temperature of 700 degrees C and annealing speed of 1 degrees C/min. The absorption coefficients of the Bi1.5:YIG films were measured to be 70 cm(-1) and 330 cm(-1), respectively, at each of the wavelengths, and the average Gilbert damping coefficient of the Bi1.5:YIG film with a Bi1:NIGG buffer layer was measured to be 6.42 +/- 18.09 x 10(-4) (with the minimum value of 0 and the maximum value of 24.51 x 10(-4)) from a conventional ferromagnetic resonance (FMR) measurement system. Our experimental result indicates that the magneto-optic property of the Bi:YIG films prepared by the MOD method is unstable and fluctuates from run to run although its average magnetic property may be useful for application to compact integrated optical isolators under an easy solution-based fabrication process.

Min-Cheol Lee,1,2 Sanghyun Lee,3 C. J. Won,4 K. D. Lee,4 N. Hur,4 Jeng-Lung Chen,5 Deok-Yong Cho,6,* and T. W. Noh1,2 1Center for Correlated Electron Systems, Institute for Basic Science, Seoul 08826, Republic of Korea 2Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea 3Institute of Material Structure Science, KEK, Tokai 319-1106, Japan 4Department of Physics, Inha University, Incheon 22212, Republic of Korea 5National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan 6IPIT and Department of Physics, Chonbuk National University, Jenju 54896, Republic of Korea

We investigated the orbital hybridization mechanism in 3d-5d double perovskites (DPs) of La2CoIrO6 and La2CoPtO6 using x-ray absorption spectroscopy. It is clearly evidenced by O K-edge and Co K-edge x-ray absorption spectra that the Co 3d orbitals hybridize not only with the half-filled Ir/Pt j(eff) states but also with the fully empty (unpolarized) Ir/Pt e(g) states in both DPs. The Co 3d e(g)-Ir 5d e(g) hybridization cannot contribute to the ferrimagnetic long-range order in La2CoIrO6 established by spin-selective Co 3d t(2g)-Ir 5d j(eff) hybridization through the intermediate oxygen p state but could serve as an origin of paramagnetism. The strengths of such orbital hybridizations were found to be almost invariant to temperature, even far above the Curie temperature, implying persistent paramagnetism against the antiferromagnetic ordering in the spin-orbit entangled 3d-5d DPs.

We investigated the crystal structure and magnetic structure in cobaltite-platinate double perovskite of La2CoPtO6, employing various techniques of X-ray diffraction, neutron diffraction, and the extended X-ray absorption fine structure analysis. It is shown that the crystal symmetry is maintained as P21/n in the temperature range of <500 °C, whereas the lattice constants and the Co-Co distances undergo a continuous structural evolution toward the high-symmetry phases with increasing temperature. The Co-O bonds were overall longer and had a larger degree of structural and thermal disorders than the Pt-O bonds. As for the magnetism, an antiferromagnetic order is stabilized in the Co2+ sublattice at temperatures below 28 K. It is demonstrated that owing to the substantial distortions (quantified by a distortion parameter Σ > 0.03 Å) of the Co-Co networks, the system is not subject to spin frustration effect. Details in the magnetic structure are determined; at 12 K, the Co magnetic moment was (0.8, 0, 2.7) Bohr magneton, the magnetic propagation vector was (-0.5, 0, 0.5), and the magnetic symmetry was preferably Γ1(Ag).

We have carried out inelastic neutron scattering experiments to study magnetic excitations in ordered double perovskite Ca$_2$FeReO$_6$. We found a well-defined magnon mode with a bandwidth of $\sim$50meV similar to previously studied Ba$_2$FeReO$_6$ below the ferri-magnetic ordering temperature $T_c\sim$520K. The spin excitations remain gapless for most of the magnetically ordered phase. However, a spin gap of $\sim$10meV opens up below $\sim$150K, which is well below the magnetic ordering temperature but coincides with the previously reported metal-insulator transition and onset of structural distortion. The observed temperature dependence of the spin gap provides strong evidence for ordering of Re orbitals at $\sim$150K, in accordance with the earlier proposal put forward by Oikawa $\it{et.\,al}$ based on neutron diffraction [J. Phys. Soc. Jpn., $\bf{72}$, 1411 (2003)] as well as the recent theoretical work by Lee and Marianetti [Phys. Rev. B, $\bf{97}$, 045102 (2018)]. The presence of separate orbital and magnetic ordering in Ca$_2$FeReO$_6$ suggests weak coupling between spin and orbital degrees of freedom and hints towards the sub-dominant role played by spin orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near the zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments, but suggests a strong damping of Re magnon mode.

Distortion of transition-metal ion-oxygen octahedra in perovskite structure has been a long standing subject for exploring the physics of charge-orbital-spin-lattice couplings. Recent findings in nickelates or BaBiO3 have suggested that a charge disproportionation in the transition-metal ions can evoke certain cooperative breathing distortions (CBD), constituted by the symmetric expansion and shrinkage of the octahedra in contrast to the Jahn-Teller-type asymmetric distortion. A double perovskite ferrimagnet, La2CoIrO6 bears inherently the charge disproportionation in the cationic sites with Co(2+ )and Ir4+ sublattices. In light of seeking the clues for the CBD and its coupling to the magnetic order, the crystal structure and magnetic structure of La2CoIrO6 were examined at various temperatures by using neutron diffraction. Structure refinements and mode decomposition analyses revealed that, indeed, a CBD mode exists substantially in La2CoIrO6 while preserving the symmetry of the lattices (P2(1)/n) and the magnetism [Gamma(1)(A(g))]. And more importantly, the magnitude of the symmetric distortion is correlated strongly with the size of the magnetic moment at temperatures below the Curie temperature (similar to 95 K). This strongly suggests a magnetoelastic coupling of the CBD mode. The order parameters for CBD, e.g., the movement of O1 toward Ir ions at low temperatures, are found to be in a linear-quadratic relationship with the value of the magnetic moment (similar to 0.002 angstrom/mu(2)(B) M-Co(2)), manifesting the magnetoelastic CBD in double perovskite.

Two dimensional layered organic-inorganic halide perovskites offer a wide variety of novel functionality such as solar cell and optoelectronics and magnetism. Self-assembly of these materials using solution process (ex. spin coating) makes crystalline thin films synthesized at ambient environment. However, flexibility of organic layer also poses a structure stability issue in perovskite thin films against environment factors (ex. moisture). In this study, we investigate the effect of solvents and moisture on structure and property in the (C6H5(CH2)(2)NH3)(2)(Cu, Mn) Cl-4 (Cu-PEA, Mn-PEA) perovskite thin films spincoated on Si wafer using three solvents (H2O, MeOH, MeOH + H2O). A combination of x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) show that relative humidity (RH) has a profound effect on perovskite thin films during sample synthesis and storage, depending on the kind of solvent used. The ones prepared using water (Cu-PEA: H2O, Mn-PEA: H2O) show quite different behavior from the other cases. According to time-dependent XRD, reversible crystalline-amorphous transition takes place depending on RH in the former cases, whereas the latter cases relatively remain stable. It also turns out from XAS that Mn-PEA thin films prepared with solvents such as MeOH and MeOH + H2O are disordered to the depth of about 4 nm from surface.

The complex structure and magnetism of Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$ was investigated by neutron scattering and EXAFS. Pr has an approximate doublet ground-state and the first excited state is a singlet. This overall crystal field level scheme is similar to metallic Pr$_2$Ir$_2$O$_7$, which is also reported here. While the B-site (Ru) is well ordered throughout, this is not the case for the A-site (Pr/Bi). A distribution of the Pr-O2 bond length indicates the Pr environment is not uniform even for $x=0$. The Bi environment is highly disordered ostensibly due to the 6s lone pairs on Bi$^{3+}$. Correspondingly we find the non-Kramers doublet ground state degeneracy otherwise anticipated for Pr in the pyrochlore structure is lifted so as to produce a quadrupolar singlet ground state with a spatially varying energy gap. For $x=0$, below T$_N$, the Ru sublattice orders antiferromagnetically, with propagation vector \textbf{k}= (0,0,0), as for Y$_2$Ru$_2$O$_7$. No ordering associated with the Pr sublattice is observed down to 100 mK. The low energy magnetic response of Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$ features a broad spectrum of magnetic excitations associated with inhomogeneous splitting of the Pr quasi-doublet ground state. For $x=0$ ($x=0.97$) the spectrum is temperature dependent (independent). It appears disorder associated with Bi alloying enhances the inhomogeneous Pr crystal field level splitting so that inter-site interactions become irrelevant for $x=0.97$. The structural complexity for the A-site may be reflected in the hysteretic uniform magnetization of B-site ruthenium in the Néel phase.

The acoustic behaviors of oxygen-reduced barium titanate (BaTiO3-delta) single crystals with delta similar to 0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO3 (BTO) showed two pronounced anomalies at approximately 112 degrees C and -11 degrees C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 degrees C compared to those of the pure BaTiO3 suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO3 were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point. (C) 2017 Elsevier B.V. All rights reserved.

Ag5Pb2O6 has attracted attentions due to its novel nearly-free-electron superconductivity, but its electronic structure and orbital character of the Cooper-pair electrons remain controversial. Here, we present a method utilizing core-level photoemission to show that Pb 6s electrons dominate near the Fermi level. We observe a strongly asymmetric Pb 4 f 7/2 core-level spectrum, while a Ag 3d 5/2 spectrum is well explained by two symmetric peaks. The asymmetry in the Pb 4 f 7/2 spectrum originates from the local attractive interaction between conducting Pb 6s electrons and a Pb 4 f 7/2 core hole, which implies a dominant Pb 6s contribution to the metallic conduction. In addition, the observed Pb 4 f 7/2 spectrum is not explained by the well-known Doniach-Šunjić lineshape for a simple metal. The spectrum is successfully generated by employing a Pb 6s partial density of states from local density approximation calculations, thus confirming the Pb 6s dominant character and free-electron-like density of states of Ag5Pb2O6.

Recently, α-RuCl3 has attracted much attention as a possible material to realize the honeycomb Kitaev model of a quantum-spin-liquid state. Although the magnetic properties of α-RuCl3 have been extensively studied, its electronic structure, which is strongly related to its Kitaev physics, is poorly understood. Here, the electronic structure of α-RuCl3 was investigated by photoemission (PE) and inverse-photoemission (IPE) spectroscopies. The band gap was directly measured from the PE and IPE spectra and was found to be 1.9 eV, much larger than previously estimated values. Local density approximation (LDA) calculations showed that the on-site Coulomb interaction U could open the band gap without spin-orbit coupling (SOC). However, the SOC should also be incorporated to reproduce the proper gap size, indicating that the interplay between U and SOC plays an essential role. Several features of the PE and IPE spectra could not be explained by the results of LDA calculations. To explain such discrepancies, we performed configuration-interaction calculations for a RuCl63− cluster. The experimental data and calculations demonstrated that the 4d compound α-RuCl3 is a Jeff = 1/2 Mott insulator rather than a quasimolecular-orbital insulator. Our study also provides important physical parameters required for verifying the proposed Kitaev physics in α-RuCl3.