The oxide superconductors, particularly those recently discovered that are based on La2CuO4have a set of peculiarities that suggest a common, unique mechanism: they tend in every case to occur near a metal-insulator transition into an odd-electron insulator with peculiar magnetic properties. This insulating phase is proposed to be the long-sought “resonating-valence-bond” state or “quantum spin liquid” hypothesized in 1973. This insulating magnetic phase is favored by low spin, low dimensionality, and magnetic frustration. The preexisting magnetic singlet pairs of the insulating state become charged superconducting pairs when the insulator is doped sufficiently strongly. The mechanism for superconductivity is hence predominantly electronic and magnetic, although weak phonon interactions may favor the state. Many unusual properties are predicted, especially of the insulating state.

http://science.sciencemag.org/content/sci/235/4793/1196.full.pdf

The resonating valence bond state in La2CuO4 and superconductivity

The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

https://iopscience.iop.org/article/10.1088/0953-8984/9/4/002/pdf

First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+ U method

Critical phenomena and renormalization-group theory

The current state of efforts to understand the phenomenon of geometric magnetic frustration is described in the context of several key materials. All are transition metal oxides which crystallize with magnetic lattices which are geometrically or topologically prone to frustration such as those based on triangles or tetrahedra which share corners, edges or faces. These include the anhydrous alums, jarosites, pyrochlores, spinels, magnetoplumbites, garnets, ordered NaCl and other structure types. Special attention is paid to materials which do not undergo long range ordering at the lowest temperatures but instead form exotic ground states such as spin glasses, spin liquids and spin ices, and to S = 1/2 based materials.

Geometrically frustrated magnetic materials

The observation of a large Nernst signal $e_N$ in an extended region above the critical temperature $T_c$ in hole-doped cuprates provides evidence that vortex excitations survive above $T_c$ The results support the scenario that superfluidity vanishes because long-range phase coherence is destroyed by thermally-created vortices (in zero field), and that the pair condensate extends high into the pseudogap state in the underdoped (UD) regime We present a series of measurements to high fields $H$ which provide strong evidence for this phase-disordering scenario

/pdf/nernst-effect-in-high-tc-superconductors-4zvz8dgk7v.pdf

Nernst effect in high-Tc superconductors

Real triangular lattice antiferromagnet with S =1/2 has been long-awaited since Anderson pointed out that its ground state might be a quantum liquid-like state rather than the Neel state. In this paper we report that NaTiO 2 and LiNiO 2 are promising candidates for it, the former being near Heisenberg-type and the latter near Ising-type. Our magnetization measurements on LiNiO 2 show a striking linear to nonlinear response transition at T N1 =210 K. A pronounced anomaly appears in ESR at another transition T N2 =∼20 K suggesting a ferrimagnet-like ordered state below. However, neither ferromagnetic nor antiferromagnetic reflections could be observed in zero field neutron scattering between 1.4 and 300 K. The susceptibility of NaTiO 2 is low suggesting a strong antiferromagnetic coupling between the spins but no Brass scattering could be observed at 1.4 K. These are suggestive of the remarkable quantum effects.

Experimental Studies of Triangular Lattice Antiferromagnets with S=1/2: NaTiO2 and LiNiO2

In order to study the frustration effects, we have taken powder diffraction patterns of VX 2 , X≡Cl, Br and I) which are thought to be quasi-two-dimensional triangular lattice antiferromagnets. The spins in VCl 2 , VBr 2 and VI 2 were found to order at 36.0, 29.5 and 16.3 K respectively forming a three sublattice structure in the basal plane. Contrary to our expectation, a partially ordered model gives a better fit to the observed diffraction rather than the Neel state with 120° structure though the spins are of Heisenberg symmetry. In VCl 2 and VBr 2 , strong diffuse scattering can be observed not only at T > T N but also at T ≪ T N . No critical scattering characteristic for 2D could be seen but the temperature variation of sublattice magnetization is rather close to that for the Ising system.

Study of Frustration Effects in Two-Dimensional Triangular Lattice Antiferromagnets–Neutron Powder Diffraction Study of VX2, X≡Cl, Br and I

Successive phase transitions of triangular lattice Heisenberg antiferromagnet with a small Ising anisotropy, CsNiCl 3 , are studied by neutron scattering We have reexamined the temperature dependences of the z and x y components of sublattice magnetizations and also measured the both components of staggered susceptibilities The character of the transitions is orderings and divergences of the z and x y components at independent temperatures

Neutron Scattering Study of Successive Phase Transitions in Triangular Lattice Antiferromagnet CsNiCl3

Critical properties of a Heisenberg antiferromagnet on a triangular lattice are investigated experimentally using a quasi-two-dimensional antiferromagnet VCl 2 by means of neutron scattering. It is found that VCl 2 shows characteristic critical behaviors of two and three dimensional models. A little above T N , the line shape of χ( Q ) is found to be represented by the Ornstein-Zernike form (κ 2 + q 2 ) -1 . This suggests the existence of magnetic point defects which were predicted in the two dimensional model. The measurecl critical exponents are β=0.20±0.02, γ=1.05±0.03, and ν=0.62±0.05, in agreement with the theoretical work for three dimensional SO(3) systems. Successive phase transitions due to a small Ising anisotropy were found at T N1 =35.88±0.01 K and T N2 =35.80±0.01 K. The spin structure and the dispersion relation of the spin wave well below T N are also determined.

Experimental Study of New Type Phase Transition in Triangular Lattice Antiferromagnet VCl2

Magnetic susceptibilities χ of the frustrated triangular lattice antiferromagnets CsVCl 3 and VX 2 (X=Cl, Br and I) have been measured by means of the conventional Faraday-type as well as the SQUID magnetometers. The χ- T curve of CsVCl 3 shows a typical 1D-like character in agreement with the early work by Niel et al . At T < T N , χ c increases rapidly with decreasing T , whereas χ a stays at a constant value. The χ- T curve in VX 2 which are thought to be 2D-like systems is very flat and shows an anomaly at T N . Surprizingly, however, no anisotropy could be detected below T N . This suggests that the transition is quite different from the conventional one for the non-frustrated antiferromagnetic systems and a magnetic liquid state is formed in the ordered state.

Magnetic susceptibilities of the frustrated triangular lattice antiferromagnets CsVCl3 and VX2 (X=Cl, Br and I): appearance of magnetic liquid in the ordered state

Koji Ubukoshi

Papers

Experimental Studies of Triangular Lattice Antiferromagnets with S=1/2: NaTiO2 and LiNiO2

Study of Frustration Effects in Two-Dimensional Triangular Lattice Antiferromagnets–Neutron Powder Diffraction Study of VX2, X≡Cl, Br and I

Neutron Scattering Study of Successive Phase Transitions in Triangular Lattice Antiferromagnet CsNiCl3

Experimental Study of New Type Phase Transition in Triangular Lattice Antiferromagnet VCl2

Magnetic susceptibilities of the frustrated triangular lattice antiferromagnets CsVCl3 and VX2 (X=Cl, Br and I): appearance of magnetic liquid in the ordered state