Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

Abstract

Layered manganese pnictides with the general formula XMnPn2 (X = Ca, Sr, Ba, Eu, or Yb, Pn = Sb, or Bi) are reported as 3D Dirac/Weyl semimetals. Although any magnetic ordering breaks the time-reversal symmetry, the specific spin structure, such as the canted antiferromagnetic structure, is required to realize the Weyl state. This dissertation is designed to investigate magnetic structures and physical properties of topological materials with the general formula XMnPn2. The compounds we have studied are BaMnSb2, EuMnSb2, and BaMnBi2.

BaMnSb2 undergoes three magnetic phase transitions at TC = 690 K, TN,2 = 450 K and TN,1 = 286 K. Below TN,1 = 286 K, Mn moments form the caned antiferromagnetic structure with spin canting angle is about 50° with respect to the c axis. The analysis of Shubnikov-de Haas oscillations reveals the low effective mass and nontrivial Berry phase. The evidence of the Zeeman splitting is also observed near n = 1. Thermal property measurements reveal the low phonon thermal conductivity and the moderate thermopower, which are important for understanding the underlying physics of BaMnSb2.

EuMnSb2 undergoes three magnetic phase transitions at TC = 700 K, TN,Mn = 346 K, and TN,Eu = 21 K. Below, Mn moments form the ferromagnetic structure below TC = 700 K and the C-type antiferromagnetic structure below TN,Mn = 346 K. Below TN,Eu = 21 K, Eu Moments form the canted A-type antiferromagnetic structure, with the spin canting angle is about 41° with respect to the a axis at 7 K. Both Mn and Eu sublattices, we obtain that the spin-spin correlation length along the a axis is much shorter than that along the b and c axes, indicating the anisotropic magnetic coupling. The analysis of the longitudinal and transverse magnetoresistance of EuMnSb2 indicates the magnetoresistance is dominated by spin scattering.

The temperature dependence of the susceptibility reversals that BaMnBi2 exhibits the antiferromagnetic transition at TN = 345 K. The temperature dependence of the in-plane resistivity shows semiconductor-like behavior with the activation energy 2.6 meV. Both MRab(H)and MRc(H)show positive magnetoresistance.

Date

7-3-2020

Committee Chair

Jin, Rongying

Available for download on Tuesday, June 29, 2027

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