Quantum magnetism and quantum phase transitions

Abstract:

Neutron diffraction and nuclear magnetic resonance (NMR) measurements have been used to analyze the magnetic structure in the S = 1 2 kagome antiferromagnet CdCu 3 ( OH ) 6 ( NO 3 ) 2 ⋅ H 2 O . Below the magnetic ordering temperature T N ≃ 4 K , magnetic Bragg reflections at (101) and (100) were found in the neutron diffraction pattern, which suggests a q = 0 magnetic structure. Furthermore, the vector spin chirality for the q = 0 structure was successfully identified from the internal field direction obtained by the 14 N -NMR measurement, previous magnetization measurements, and magnetic symmetry analysis. Our findings point to a chirality-ordered magnetic structure with negative vector chirality and ⟨ 100 ⟩ anisotropy.

Abstract:

Dimensionality is a critical factor in determining the properties of solids and is an apparent built-in character of the crystal structure. However, it can be an emergent and tunable property in geometrically frustrated spin systems. Here, we study the spin dynamics of the tetrahedral cluster antiferromagnet, pharmacosiderite, via muon spin resonance and neutron scattering. We find that the spin correlation exhibits a two-dimensional characteristic despite the isotropic connectivity of tetrahedral clusters made of spin 5/2 Fe3+ ions in the three-dimensional cubic crystal, which we ascribe to two-dimensionalisation by geometrical frustration based on spin wave calculations. Moreover, we suggest that even one-dimensionalisation occurs in the decoupled layers, generating low-energy and one-dimensional excitation modes, causing large spin fluctuation in the classical spin system. Pharmacosiderite facilitates studying the emergence of low-dimensionality and manipulating anisotropic responses arising from the dimensionality using an external magnetic field.