Studies of charge density wave phenomena in alkali metals

Yong Gyoo Hwang, Purdue University

Abstract

The neutron-diffraction determination of the Charge Density Wave (CDW) wavevector $\vec{\rm Q}$ in potassium is used to calculate the detailed structure of the conduction-electron Fermi surface. A Schrodinger equation having two periodic potentials, Gcos$\vec{\rm Q}{\cdot}\vec{\rm r}$ from the CDW, and Bcos$\vec{\rm G}{\cdot}\vec{\rm r}$ from the lattice, is solved numerically. Minigaps in E($\vec{\rm k}$), caused by perturbations having periodicity (n + 1)$\vec{\rm Q}$ $-$ n$\vec{\rm G}$, n = 1, 2,$\cdots$, lead to small cylindrical sections of Fermi surface. These cylinders give rise to perpendicular field cyclotron resonance and unexpected angle-resolved photoemission peaks. Heterodyne gaps, having periodicity n($\vec{\rm G}$ $-$ $\vec{\rm Q}$), are created in the equatorial region of the Fermi "sphere". Open-orbit magnetoresistance peaks observed by Coulter and Datars (in K and Na) arise from the multiply-connected Fermi-surface topology caused by the minigaps and heterodyne gaps. A spin-wave theory is developed for alkali metals having a CDW structure. For simplicity the spin-dependent part of the many-body interaction between quasi-particles is taken as a constant, thereby neglecting Landau parameters B$\sb{\rm n}$ with n $\geq$ 1. In a CDW state the velocity distribution is not only anisotropic but open orbit motion becomes possible. As a result, the motion of an electron is significantly modified. Since the known properties of alkali metals indicate CDW structure, the interpretation of experimental spin-wave data is reconsidered. The revised values of the Landau parameters B$\sb0$ and the fraction of electrons in open orbits $\eta$ for potassium are found to be B$\sb0$ = $-0.252$($\pm$0.003) and $\eta$ = 0.048($\pm$0.015) when only the open orbit effect is included. B$\sb0$ = $-0.221$($\pm$0.002) and $\eta$ = 0.042($\pm$0.013) if Fermi surface distortion is also included. Also it is shown that observed splittings of spin-wave side bands can be explained when effects of CDW domain structure are recognized.

Degree

Ph.D.

Advisors

Overhauser, Purdue University.

Subject Area

Condensation

COinS