A new quasi-one-dimensional molecular solid based on Ni(mnt)2 anion: Crystal structure and spin-gap transition

A new molecular solid, [1-(4′-bromo-2′-fluorobenzyl)-4-dimetylaminopyridinium]-bis(maleonitriledithiolato)nickel(III), (BrFBzPyN(CH₃)₂(Ni(mnt)₂)(1), has been prepared and characterized by elemental analyses, IR, ESI-MS spectra, single crystal X-ray diffraction and magnetic measurements. Compound 1 crystallizes in the orthorhombic space group Pnma, a=20.579(4) Å, b=7.078(1) Å, c=17.942(4) Å, α=β=γ=90°, V=2613.3(9) Å³, Z=4. The Ni(III) ions of 1 form a quasi-one-dimensional Zigzag magnetic chain within a Ni(mnt)₂⁻ column through Ni?S, S?S, Ni?Ni, or π?π interactions with an Ni?Ni distance of 4.227 Å. Magnetic susceptibility measurements in the temperature range 2-300 K show that 1 exhibits a spin-gap transition around 200 K, and antiferromagnetic interaction in the high-temperature phase (HT) and spin gap in the low-temperature phase (LT). The transition for 1 is second-order phase transition as determined by DSC analyses.     

Switching between one- and two-dimensional conductance: Coupled chains in the monolayer of Pb on Si(5 5 7)

Four-point conductance measurements of a high temperature annealed monolayer of Pb on Si(5 5 7) are combined with tunneling microscopy (STM) and LEED for structural investigations. We found extremely high surface state conductance which becomes quasi-one-dimensional below a critical temperature of T_c = 78 K. The change from low to extremely high conductance anisotropy is associated with a reversible order-disorder phase transition with a temperature dependence ∝1/T + const. along the chains below the phase transition. Below the phase transition order is found simultaneously in the lateral separation between Pb chains and along the chains. There a 10-fold superperiodicity appears. We suggest that strong two-dimensional coupling, leading to electronic stabilization of terraces with a width of lattice constants, results in perfect nesting normal to the chains at E_F (below T_c), which is the origin of this switching to one-dimensional behavior. Therefore, no metal-insulator transition is expected at low temperature.     

Infrared laser magnetic resonance spectroscopy of the ν3 fundamental and associated hot bands of the NCO free radical

The v₃ = 1 ← 0 (out-of-phase stretching vibration) transition wavenumbers of gas-phase NCO have been measured, in many cases with sub-Doppler resolution, by mid-infrared laser magnetic resonance (LMR) spectroscopy. In addition to the fundamental transition from the ²Π ground vibronic state, hot bands from the v₂ = 1 ²Σ and ²Δ and from the v₂ = 2 ²Φ vibronic states were detected and analyzed. The ²Σ vibronic level, with a spin-orbit coupling only partly quenched by the Renner-Teller effect, is characterised by complex and rapid tuning of energies in the magnetic field. This is the first successful analysis of magnetic resonance spectra for non-unique Renner-Teller vibronic states. The new LMR transitions were combined with data from previous studies and analysed, using an effective Hamiltonian in a 40-parameter fit to 660 transitions and combination differences. Several new coupling parameters are required, having magnitudes generally consistent with predictions from standard vibration-rotation theory.     

Spin-Peierls and incommensurate states in layered S=1/2 system TiOCl

We demonstrate that TiOCl is a good model inorganic system to investigate spin-Peierls state. Our ³⁵Cl and ^47,49Ti NMR data show that a pseudo spin-gap behavior below T*=135 K precedes successive phase transitions at T_c=94 K and into a singlet spin-Peierls ground state with a large energy gap E_g/k_B=430 K.     

A holographic model of deconfinement and chiral symmetry restoration

We analyze the finite temperature behavior of the Sakai-Sugimoto model, which is a holographic dual of a theory which spontaneously breaks a U(N_f)_L × U(N_f)_R chiral flavor symmetry at zero temperature. The theory involved is a 4 + 1 dimensional supersymmetric SU(N_c) gauge theory compactified on a circle of radius R with anti-periodic boundary conditions for fermions, coupled to N_f left-handed quarks and N_f right-handed quarks which are localized at different points on the compact circle (separated by a distance L). In the supergravity limit which we analyze (corresponding in particular to the large N_c limit of the gauge theory), the theory undergoes a deconfinement phase transition at a temperature T_d = 1/2πR. For quark separations obeying L > L_c ? 0.97 ∗ R the chiral symmetry is restored at this temperature, but for L < L_c ? 0.97 ∗ R there is an intermediate phase which is deconfined with broken chiral symmetry, and the chiral symmetry is restored at T_χSB ? 0.154/L. All of these phase transitions are of first order.     

Magnetic-field-driven quantum critical behavior in graphite and bismuth

We study magnetotransport properties of graphite and rhombohedral bismuth samples and found that in both materials applied magnetic field induces the metal-insulator- (MIT) and reentrant insulator-metal-type (IMT) transformations. The corresponding transition boundaries plotted on the magnetic field-temperature (B − T) plane nearly coincide for these semimetals and can be best described by power laws T ∼ (B − B_c)^κ, where B_c is a critical field at T = 0 and κ = 0.45 ± 0.05. We show that insulator-metal-insulator (I-M-I) transformations take place in the Landau level quantization regime and illustrate how the IMT in quasi-3D graphite transforms into a cascade of I-M-I transitions, related to the quantum Hall effect in quasi-2D graphite samples. We discuss the possible coupling of superconducting and excitonic correlations with the observed phenomena, as well as signatures of quantum phase transitions associated with the M-I and I-M transformations.     

Magnetic phase transitions in Nd7Rh3 single crystal

Magnetic phase transitions in rare earth intermetallic compound Nd₇Rh₃ have been investigated using a single crystal. Measurement results of magnetization, magnetic susceptibility, specific heat, and electrical resistivity reveal that Nd₇Rh₃ has two magnetic phase transitions at T_N=34 K, T_t2=9.1 K and a change of the magnetic feature at T_t1=6.8 K in the absence of an external magnetic field. Antiferromagnetic orderings exist in all the three magnetic states; a large magnetic anisotropy between the c-axis and the c-plane is observed. In the magnetic phase below T_t2, an irreversible field-induced magnetic phase transition takes place in the c-plane; after removing external magnetic field, a coexistence state of ferro- and antiferromagnetic ordering or a ferrimagnetic state having a remanent magnetization M_R is stabilized. The M_R decays to a certain value for several hours after the first process; a magnetic field cooling effect was also observed in the c-plane below T_t2. In the antiferromagentic state above T_t2, the irreversibility disappears and an ordinary antiferromagnetic state takes place. As the origin of this phenomenon, a kind of martensitic structural transition that is observed in Gd₅Ge₄ can be considered.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

Molecular dynamics of [(CH2OH)3CNH3]2SiF6 by phase transition of 178 K

We have investigated the molecular motions of TRIS⁺ ([(CH₂OH)₃CNH₃]⁺) and ions in the [(CH₂OH)₃CNH₃]₂SiF₆ crystal below room temperature from the measurements of the spin-lattice relaxation time T₁ and the NMR absorption line of ¹H and ¹⁹F nuclei, in order to elucidate the changes of the molecular motions by the phase transition of T_c=178 K. The narrowing of the ¹⁹F-NMR line was observed around T_c=178 K and the reorientation of the anion appears above T_c. Moreover, from the analysis of the temperature dependence of T₁, we have observed that the activation energy of the reorientational motion of ions changes from 0.168 eV (T>T_c) to 0.185 eV (T<T_c). Based on these results, we found that the reorientational motion of ions is closely related to the origin of the phase transition at T_c. In addition, from the measurement of the ¹H-NMR line, we also found that the reorientational motion of H₂ in the -CH₂OH group becomes active accompanied by the phase transition.     

Structural phase transitions at high-temperature in double perovskite Sr2GdRuO6

The crystal structure evolution of the Sr₂GdRuO₆ complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K≤T≤1273 K. Powder X-ray diffraction measurements at room temperature and Rietveld analysis show that this compounds crystallizes in a monoclinic perovskite-type structure with P2₁/n (#14) space group and the 1:1 ordered arrangement of Ru⁵⁺ and Gd³⁺ cations over the six-coordinate M sites, with lattice parameters a=5.81032(8) Å, b=5.82341(4) Å, c=8.21939(7) Å, V=278.11(6) Å³ and angle β=90.311(2)^o. The high-temperature analysis shows that this material suffers two-phase transitions. At 373 K it adopts a monoclinic perovskite structure with I2/m space group, and lattice parameters a=5.81383(2) Å, b=5.82526(4) Å, c=8.22486(1) Å, V=278.56(2) Å³ and angle β=90.28(2)^o. Above of 773 K, it suffers a phase transition from monoclinic I2/m to tetragonal I4/m, with lattice parameters a=5.84779(1) Å, c=8.27261(1) Å, V=282.89(5) Å³ and angle β=90.02(9)^o. The high-temperature phase transition from monoclinic I2/m to tetragonal I4/m is characterized by strongly anisotropic displacements of the anions.     

Metamagnetic transition and magnetocaloric effect in antiferromagnetic TbPdAl compound

Magnetic properties and the magnetocaloric effect of the compound TbPdAl are investigated. The compound exhibits a weak antiferromagnetic (AFM) coupling, and undergoes two successive AFM transitions at T_N=43 K and T_t=22 K. A field-induced metamagnetic transition from AFM to ferromagnetic (FM) state is observed below T_N, and a small magnetic field can destroy the AFM structure of TbPdAl, inducing an FM-like state. The maximal value of magnetic entropy change is −11.4 J/kg K with a refrigerant capacity of 350 J/kg around T_N for a field change of 0-5 T. Good magnetocaloric properties of TbPdAl result from the high saturation magnetization caused by the field-induced AFM-FM transition.     

Lifetimes and transition frequencies of several singlet ungerade states in N2 between 106 000 and 109 000 cm

Using a narrow-band tunable XUV source, ultra-high resolution 1 XUV + 1 UV two-photon ionisation spectra were recorded of transitions to several singlet ungerade states in ¹⁴N₂ and ¹⁵N₂ in the range 106 000-109 000 cm⁻¹. The natural linewidths of the individual rotational spectral lines were determined and the resulting lifetimes were found to depend on vibrational level and for the c₃¹Π_u (v = 1) level also on isotope. Furthermore, accurate transition frequencies were determined and for several bands, lines near bandhead regions were resolved for the first time.     

Bloch-Ising phase transition in spin solitons

Heisenberg spin chains with ferromagnetic nearest-neighbor coupling and an easy axis of magnetisation are considered in the classical limit. The critical ratioa _c between the anisotropy coefficient and the exchange integral for which the 180°-Blochtype soliton undergoes a phase transition to an Ising-type soliton is calculated by an exact analytical algorithm. For chains including 2N=4,6,8,... spins, one finds a sequencea _c (2N) which converges rapidly to the valuea _c()=2/3 (e.g.a _c (12)=0.66664) marking the phase transition from a Bloch- to an Ising-structure of the soliton in an infinite chain.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

The pure rotational spectrum of Difluoroiodomethane, CHF2I

The pure rotational spectrum of CHF₂I has been recorded for the first time, in a supersonic expansion in the region 1.7-17 GHz, and at room-temperature in the region 302-318 GHz. The observed transitions span the values of J^″ from 0 up to 67. Precise rotational and centrifugal distortion constants have been determined. Furthermore, the complete iodine nuclear electric quadrupole coupling tensor, in the inertial and principal axes, has been determined. Quantum chemical calculations have been performed to aid with the analysis. Iodine quadrupole mediated perturbations have resulted in the following observations: (i) several transitions having enhanced intensities and (ii) the observation of several forbidden, ΔJ=±2, transitions. Comparisons in electronic structure are made between the series of molecules CH_3-nF_nX; and X = Cl, Br, I.     

Coexistence of ferromagnetism and superconductivity in NiBi-binary alloy

We investigate the effects of the antiferromagnetic exchange coupling between the Nickel and Bismuth2 atoms (J_int2 < 0) on the magnetizations of the NiBi-binary alloy versus temperature and external magnetic field by means of the effective field theory. We find that the magnetization of the Ni, Bi1, Bi2 and total NiBi-binary alloy has two different magnetic phase transitions for the J_int2 < 0. One of them is a first-order phase transition (FOPT) at T_t = 0.349 and the other is a second-order phase transition (SOPT) at T_c = 0.791. We also study the hysteresis behaviors and we find that the values of the coercive field points of the Bi2 are higher than those of the Ni and Bi1. Moreover, Ni, Bi1 and Bi2 components have ferromagnetic hysteresis behaviors whereas the total NiBi has type II superconducting behavior. Therefore, we suggest that ferromagnetism and superconductivity coexist in the NiBi-binary alloy that is qualitatively good agreement with the some experimental and theoretical works.     

Effect of Mn-doping on the electrical properties of Cu2GeSe3

In this work we report the temperature dependence of the resistivity ρ of p-Cu₂GeSe₃ and manganese-doped p-Cu₂GeSe₃ at low temperature. It was found that for a intrinsic sample ρ obeys the Shklovskii-Efros-type variable-range hopping resistivity law in the temperature range from 4 to 63 K. This behaviour is governed by generation of a Coulomb gap Δ=78 meV in the density of localized states. We find a low activation term T₀=0.24 K, which is an indication of a large localization length ξ. For Mn-doped sample a metal-insulator transition (MIT) is observed at T=65 K. On the basis of the Mott criterion for metal-insulator transition, the critical carrier density n_c is determined. From the analysis of resistivity data it is concluded that Mn acts as acceptor impurity.