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Register a new userStudy of one-dimensional nature of (Sr,Ba)_2Cu(PO_4)_2 and BaCuP_2O_7 via 31P NMR
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The magnetic behavior of the low-dimensional phosphates (Sr,Ba)_2 Cu(PO_4)_2 and BaCuP_2O_7 was investigated by means of magnetic susceptibility and ^{31}P nuclear magnetic resonance (NMR) measurements. We present here the NMR shift K(T), the spin-lattice 1/T_1 and spin-spin 1/T_2 relaxation-rate data over a wide temperature range 0.02 K < T < 300 K. The T-dependence of the NMR K(T) is well described by the S=1/2 Heisenberg antiferromagnetic chain model with an intrachain exchange of J/k_B = 165 K, 151 K, and 108 K in Sr_2Cu(PO_4)_2, Ba_2Cu(PO_4)_2, and BaCuP_2O_7, respectively. Our measurements suggest the presence of magnetic ordering at 0.8 K in BaCuP_2O_7 (J/k_B = 108 K). For all the samples, we find that 1/T_1 is nearly T-independent at low-temperatures (1 K < T < 10 K), which is theoretically expected for 1D chains when relaxation is dominated by fluctuations of the staggered susceptibility. At high temperatures, 1/T_1 varies nearly linearly with temperature.
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The magnetic properties of Na2CuP2O7 were investigated by means of 31P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the 31P NMR shift, the spin-lattice 1/T1, and spin-spin 1/T2 relaxation-rate data as a function of temperature T. The temperature dependence of the NMR shift K(T) is well described by the S=1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an intraplanar exchange of J/k_B simeq 18pm2 K and a hyperfine coupling A = (3533pm185) Oe/mu_B. The 31P NMR spectrum was found to broaden abruptly below T sim 10 K signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data down to 1.8 K. The heat capacity appears to have a weak maximum around 10 K. With decrease in temperatures, the spin-lattice relaxation rate 1/T1 decreases monotonically and appears to agree well with the high temperature series expansion expression for a S = 1/2 2D square lattice.
We have employed first principles calculations to study the electronic structure and magnetic properties of the low-dimensional phosphates, Ba2Cu(PO4)2 and Sr2Cu(PO4)2. Using the self-consistent tight-binding lin- earized muffin-tin orbital method and the Nth order muffin-tin orbital method, we have calculated the various intrachain as well as the interchain hopping parameters between the magnetic ions Cu2+ for both the com- pounds. We find that the nearest-neighbor intrachain hopping t is the dominant interaction suggesting the compounds to be indeed one dimensional. Our analysis of the band dispersion, orbital projected band struc- tures, and the hopping parameters confirms that the Cu2+-Cu2+ super-super exchange interaction takes place along the crystallographic b direction mediated by O-P-O. We have also analyzed in detail the origin of short-range exchange interaction for these systems. Our ab initio estimate of the ratio of the exchange inter- action of Sr2Cu(PO4)2 to that of Ba2Cu(PO4)2 compares excellently with available experimental results.
Quasi-one-dimensional iron-based ladders and chains, with the 3$d$ iron electronic density $n = 6$, are attracting considerable attention. Recently, a new iron chain system Ba$_2$FeS$_3$, also with $n = 6$, was prepared under high-pressure and high-temperature conditions. Here, the magnetic and electronic phase diagrams are theoretically studied for this quasi-one-dimensional compound. Based on first-principles calculations, a strongly anisotropic one-dimensional electronic band behavior near the Fermi level was observed. In addition, a three-orbital electronic Hubbard model for this chain was constructed. Introducing the Hubbard and Hund couplings and studying the model via the density matrix renormalization group (DMRG) method, we studied the ground-state phase diagram. A robust staggered $uparrow$-$downarrow$-$uparrow$-$downarrow$ AFM region was unveiled in the chain direction, consistent with our density functional theory (DFT) calculations. Furthermore, at intermediate Hubbard $U$ coupling strengths, this system was found to display an orbital selective Mott phase (OSMP) with one localized orbital and two itinerant metallic orbitals. At very large $U/W$ ($W$ = bandwidth), the system displays Mott insulator characteristics, with two orbitals half-filled and one doubly occupied. Our results for high pressure Ba$_2$FeS$_3$ provide guidance to experimentalists and theorists working on this one-dimensional iron chalcogenide chain material.
Rb-NMR study has been performed on the quasi-one dimensional competing spin chain Rb2Cu2Mo3O12 with ferromagnetic and antiferromagnetic exchange interactions on nearest neighboring and next nearest neighboring spins, respectively. The system changes from a gapped ground state at zero field to the gapless state at H_C simeq 2 T, where the existence of magnetic order below 1 K was demonstrated by a broadening of NMR spectrum, associated with a critical divergence of 1/T_1. In higher temperature region, 1/T_1 showed a power-law type temperature dependence, from which the field dependence of Luttinger parameter K was obtained and compared with theoretical calculations based on the spin nematic Tomonaga Luttinger Liquid (TLL) state.
Magnetic properties in the quasi-one-dimensional organic salt (TMTTF)2SbF6 are investigated by 13C NMR under pressures. Antiferromagnetic phase transition at ambient pressure (AFI) is confirmed. Charge-ordering is suppressed by pressure and is not observed under 8 kbar. For 5 < P < 20 kbar, a sharp spectrum and the rapid decrease of the spin-lattice relaxation rate 1/T1 were observed below about 4 K, attributed to a spin-gap transition. Above 20 kbar, extremely broadened spectrum and critical increase of 1/T1 were observed. This indicates that the system enters into another antiferromagnetic phase (AFII) under pressure. The slope of the antiferromagnetic phase transition temperature T_AFII, dT_AFII/dP, is positive, while T_AFI decreases with pressure. The magnetic moment is weakly incommensurate with the lattice at 30 kbar.
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