Discovery of a spin-singlet ground state with an energy gap in CaCuGe2O6

The Fascinating World of Low-Dimensional Quantum Spin Systems: Ab Initio Modeling

In recent times, ab initio density functional theory has emerged as a powerful tool for making the connection between models and materials. Insulating transition metal oxides with a small spin forms a fascinating class of strongly correlated systems that exhibit spin-gap states, spin–charge separation, quantum criticality, superconductivity, etc. The coupling between spin, charge, and orbital degrees of freedom makes the chemical insights equally important to the strong correlation effects. In this review, we establish the usefulness of ab initio tools within the framework of the N-th order muffin orbital (NMTO)-downfolding technique in the identification of a spin model of insulating oxides with small spins. The applicability of the method has been demonstrated by drawing on examples from a large number of cases from the cuprate, vanadate, and nickelate families. The method was found to be efficient in terms of the characterization of underlying spin models that account for the measured magnetic data and provide predictions for future experiments.

Three different Ge environments in a new Sr5CuGe9O24 phase synthesized at high pressure and high temperature

In the framework of expanding the range of copper-based compounds in the pyroxene family, we have synthesized at high pressure and high temperature a powder containing a mixture of a new phase with stoichiometry Sr₅CuGe₉O₂₄ having two identified impurity phases. Electron crystallography showed that the new phase crystallizes in the monoclinic space group P2/c, with unit-cell parameters a = 11.8 Å, b = 8.1 Å, c = 10.3 Å and β = 101.3°. We applied the recently developed low-dose electron diffraction tomography method to solve the structure by direct methods. The obtained structure model contains all 9 independent cation positions and all 13 oxygen positions. A subsequent refinement against powder X-ray diffraction data ascertained the high quality of the structure solution, in particular, the unusual structural arrangement that there are three different environments for Ge in this phase.

Two spin-canted antiferromagnetic orderings and a field-induced metamagnetic transition in SrMn2(VO4)2(H2O)2

A new vanadate SrMn2(VO4)2(H2O)2 was synthesized and characterized by single-crystal X-ray analysis. It crystallizes in the monoclinic system with space group C2/m, and exhibits a quasi-one-dimensional (1D) structure. The Mn2+ ions form infinite linear-chains along the b-axis through edge-sharing oxygen atoms and further the linear-chains are connected by VO4 polyhedra into a layer in the ab-plane with Sr2+ ions residing in the space between the layers. Magnetic measurement results show that SrMn2(VO4)2(H2O)2 possesses two spin-canted antiferromagnetic orderings at ∼45 K and ∼7 K. The first ordering at ∼45 K possibly corresponds to a small deviation of spins from a strictly antiparallel arrangement. As the temperature decreases, further spin rotations occur leading to a steadier noncollinear antiferromagnetic phase, leading to the second ordering at ∼7 K. Also, a field-induced transition is observed at a critical field (0.4 T) below ∼7 K.

Intra-chain superexchange couplings in quasi-1D 3d transition-metal magnetic compounds

The electronic structure and magnetic properties of the quasi-1D transition-metal borates PbMBO4 (M  =  Ti, V, Cr, Mn, Fe, Co) have been investigated by density functional theory, including electronic correlation. The results evidence PbCrBO4 and PbFeBO4 as antiferromagnetic (AFM) semiconductors (intra-chain AFM and inter-chain FM) and PbMnBO4 as a ferromagnetic (FM) semiconductor (both intra- and inter-chain FM) in accordance with experimental observations. For non-synthesized PbTiBO4, PbVBO4, and PbCoBO4, the ground-state magnetic structures are paramagnetic, FM, and paramagnetic, respectively. In this series of compounds, there are two kinds of superexchange couplings dominating their magnetic properties, i.e. the direction M-M delocalization superexchange and indirect M-O-M correlation superexchange. For PbMBO4 with M (3+) d  (n) , n  ⩽  3 (M  =  V and Cr), the main intra-chain spin coupling is the M-M t 2g-t 2g direct delocalization superexchange, while for PbMBO4 with M (3+) d  (n) , n  >  3 (M  =  Mn and Fe), the main intra-chain spin coupling is the near 90° M-O-M e g-p-e g indirect correlation superexchange.

Synthesis, characterization, and mechanism analysis of S = 2 quasi-one-dimensional ferromagnetic semiconductor Pb2Mn(VO4)2(OH)

A brackebuschite-type compound Pb2Mn(VO4)2(OH) was synthesized by a hydrothermal method. Single crystal X-ray diffraction reveals that Pb2Mn(VO4)2(OH) crystallizes in the space group P21/m, with the structure built of single [010] chains of edge-shared MnO6 octahedra bridged by VO4-PbO6 groups along a and c axes, as a result forming a very large inter-chain distance of about 7.67 Å. The magnetic and optical measurements indicate that Pb2Mn(VO4)2(OH) is an S = 2 quasi-one-dimensional ferromagnetic semiconductor, with a very small specific value of intra- and inter-chain exchange coupling Jintra/Jinter = 10(-3), and a band gap of 1.72 eV. The electronic structure calculations indicate that a 90° Mn-O-Mn dpσ-dpσ correlation superexchange dominates the intra-chain ferromagnetic coupling in Pb2Mn(VO4)2(OH).

Two S = 1/2 one-dimensional barium copper phosphates showing antiferromagnetic and ferromagnetic intrachain interactions

Two barium copper phosphates, BaCu2(PO4)2(H2O) (1) and Ba2Cu(HPO4)(PO4)(OH) (2), were synthesized under mild hydrothermal conditions. The Cu cation in 1 adopts a CuO4(H2O) square pyramidal coordination configuration, forming alternating chains along the b axis through alternative corner and edge sharing, while the geometry of the Cu center in 2 is a CuO4(OH)2 octahedron which further connects each other by edge sharing to constitute uniform chains along the b axis. Magnetic behaviors of both compounds were analyzed by susceptibility, magnetization and heat capacity measurements. The dominant intrachain couplings are antiferromagnetic in 1 with a long-range ordering at 14 K and ferromagnetic in 2 without long-range ordering above 2 K. The first principles calculations indicate that the intrachain ferromagnetic couplings in 2 originate from Cu(1)-O(7)H-Cu(1) dpσ correlation superexchanges. The susceptibility data of compounds 1 and 2 are fitted by using suitable antiferromagnetic chain and ferromagnetic chain models, respectively. In addition, we report the results of the infrared and thermal measurements of both the compounds.

Spin liquids and antiferromagnetic order in the Shastry-Sutherland-lattice compound Yb2Pt2Pb

We present measurements of the magnetic susceptibility χ and the magnetization M of single crystals of metallic Yb(2)Pt(2)Pb, where localized Yb moments lie on the dimerized and frustrated Shastry-Sutherland lattice (SSL). Strong magnetic frustration is found in this quasi-two-dimensional system, which orders antiferromagnetically at T(N) = 2.02 K from a paramagnetic liquid of Yb dimers, having a gap Δ = 4.6 K between the singlet ground state and the triplet excited states. Magnetic fields suppress the antiferromagnetic (AF) order, which vanishes at a 1.23 T quantum critical point. The spin gap Δ persists to 1.5 T, indicating that dimer singlets survive the collapse of the B = 0 AF state. Quantized steps are observed in M(B) within the AF state, a signature of SSL systems. Our results show that Yb(2) Pt(2)Pb is unique, both as a metallic SSL system that is close to an AF quantum critical point, and as a heavy fermion compound where quantum frustration plays a decisive role.

Unusually large magnetic anisotropy in a CuO-based semiconductor Cu5V2O10

A CuO-based material Cu(5)V(2)O(10) was successfully grown in a closed crucible using Sr(OH)(2)·8H(2)O as flux. The structure of Cu(5)V(2)O(10) can be viewed as being composed of two types of zigzag Cu-O chains running along the b- and c-axes, which shows a two-dimensional crosslike framework with 12-column square tunnels along the a-axis. Magnetic measurements show that Cu(5)V(2)O(10) exhibits unexpected large magnetic anisotropy, which is the first time magnetic anisotropy energy of ∼10(7) erg/cm(3) in the CuO-based materials has been observed. The origins of large anisotropy are suggested to arise from strong anisotropic exchanges due to the particular bonding geometry and the Jahn-Teller distortion of Cu(2+) ions. Further, the band structure investigated by the GGA+U method suggests that Cu(5)V(2)O(10) is a semiconductor.

Investigation of the Crystal Structure and the Structural and Magnetic Properties of SrCu2(PO4)2

SrCu2(PO4)2 was prepared by the solid-state method at 1153 K. Its structure was solved by direct methods in the space group Pccn (No. 56) with Z = 8 from synchrotron X-ray powder diffraction data measured at room temperature. Structure parameters were then refined by the Rietveld method to obtain the lattice parameters, a = 7.94217(8) A, b = 15.36918(14) A, and c = 10.37036(10) A. SrCu2(PO4)2 presents a new structure type and is built up from Sr2O16 and Cu1Cu2O8 units with Cu1...Cu2 = 3.256 A. The magnetic properties of SrCu2(PO4)2 were investigated by magnetic susceptibility, magnetization up to 65 T, Cu nuclear quadrupole resonance (NQR), electron-spin resonance, and specific heat measurements. With spin-dimer analysis, it was shown that the two strongest spin-exchange interactions between Cu sites result from the Cu1-O...O-Cu2 and Cu2-O...O-Cu2 super-superexchange paths with Cu1...Cu2 = 5.861 A and Cu2...Cu2 = 5.251 A, and the superexchange associated with the structural dimer Cu1Cu2O8 is negligible. The magnetic susceptibility data were analyzed in terms of a linear four-spin cluster model, Cu1-Cu2-Cu2-Cu1 with -2J(1)/kB = 82.4 K for Cu1-Cu2 and -2J(2)/k(B) = 59 K for Cu2-Cu2. A spin gap deduced from this model (Delta/kB = 63 K) is in agreement with that obtained from the Cu NQR data (Delta/kB = 65 K). A one-half magnetization plateau was observed between approximately 50 and 63 T at 1.3 K. Specific heat data show that SrCu2(PO4)2 does not undergo a long-range magnetic ordering down to 0.45 K. SrCu2(PO4)2 melts incongruently at 1189 K. We also report its vibrational properties studied with Raman spectroscopy.

Importance of Supersuperexchange Interactions in Determining the Dimensionality of Magnetic Properties. Determination of Strongly Interacting Spin Exchange Paths in A2Cu(PO4)2 (A = Ba, Sr), ACuP2O7 (Ba, Ca, Sr, Pb), CaCuGe2O6, and Cu2UO2(PO4)2 on the Basis of Qualitative Spin Dimer Analysis

The patterns of the Cu(2+) ion arrangements in the magnetic oxides A(2)Cu(PO(4))(2) (A = Ba, Sr), ACuP(2)O(7) (Ba, Ca, Sr, Pb), CaCuGe(2)O(6), and Cu(2)UO(2)(PO(4))(2) are quite different from the patterns of the strongly interacting spin exchange paths deduced from their magnetic properties. This apparently puzzling observation was explained by evaluating the strengths of the Cu-O-Cu superexchange and Cu-O...O-Cu supersuperexchange interactions of these oxides on the basis of qualitative spin dimer analysis. Supersuperexchange interactions are found to be crucial in determining the dimensionality of magnetic properties of these magnetic oxides.