Research On Electronic Structure,Magnetic Properties And Transport Properties Of Heavy-Rare-Earth-Based Half-Heusler Alloys

The discover of quantum spin Hall Effect and corresponding novel quantum effect in 1980 s opened a new era of research on topological materials.Because of its novel properties and potential application,the predication and researches of topological materials become hot topic in condensed matter systems.Under the efforts of many researchers,several classifications of topological phase were developed.According to the feature of band,these materials can be divided into topological insulator,Dirac semimetal,Weyl semimetal,topological superconductors and topological nodal line semimetal,and so on.As a materials with the broken of time inversion symmetry or spatial inversion symmetry,Weyl semimetal host abundant physical properties like chiral-anomaly-induced negative magnetoresistance,planar Hall effect,non-tivial Berry Phase,giant positive magnetoresistance,and so on.Magnetic Weyl semimetal attracts people's attentions for their more abundant and excellent properties.Half-Heusler alloys are ancient highly ordered compounds.The composted elements widely distribute in periodic table of elements that makes the compounds have a variety of combinations.A lot of nonmagnetic half-Heusler compounds with 18 valance electron were predicted to topological insulator candidate for their similar crystal structure with Hg Te.Many physical properties like large magnetoresistance,linear magnetoresistance,superconduction and Weak antilocalization were observed in these compounds.Magnetic half-Heusler alloys are considered to be magnetic-field-induced Weyl semimetal in which large anomalous Hall Effect and chiral anomaly were observed.In this doctoral thesis,the research object is on the heavy-rare-earth half-Heusler alloys RPt Bi(R=Tb,Dy,Ho,Er and Tm)with 4f electrons.The novel magnetic transport properties and tune effect of 4f electrons were studied.The main contents and innovation points of the paper are as follows:1)The identification of topological electronic structure in RPt Bi(R=Tb,Ho,Er)alloys.The first principle calculation on Tb Pt Bi,Ho Pt Bi amd Er Pt Bi show that three compounds host semimetal band that only a tiny band across the Fermi level on ? point,the center of Brillouin zone.Then,the band near Fermi level was opened when the SOC was considered.The Weyl points may hide in the non-symmetric path.It indicated that half-Heusler alloys are a kind of potential Weyl semimetal.2)Chiral-anomaly-induced negative magnetoresistance in half-Heusler alloys RPt Bi(R=Tb,Ho,Er).A series of high quality single crystals of RPt Bi were synthesized by using excess Bi as the flux.Then,the angular dependent magnetoresistance indicates that negative is sensitive to the angle between B and I.The current jetting effect as the origin of negative magnetoresistance was excluded by designed experimental measurement.It identified that chiral anomaly is the origin of negative magnetoresistance;We also make a analysis on the principle of negative magnetoresistance;In low temperature and low magnetic field,Er Pt Bi shows a large planar Hall Effect and the origin of this effect was also discussed;Non-trivial Berry Phase ? was extracted from Sd H oscillation in Ho Pt Bi.All of these evidences of transport properties combined with first principle calculation identified that Tb Pt Bi,Ho Pt Bi and Er Pt Bi maybe the magnetic-field-induced Weyl semimetal.3)Topological enhanced anomalous Hall effect in half-Heusler alloys RPt Bi(R=Tb,Ho)alloys.The measurement on Hall Effect implied that Tb Pt Bi shows a large anomalous Hall Effect when the magnetic field B>7T.The largest Hall angle can reached to 32% that is largest in typical systems with anomalous Hall Effect.Different from Tb Pt Bi,Ho Pt Bi not only host the large anomalous Hall Effect,but also show a anisotropic anomalous Hall Effect.The anomalous Hall resistivity shows opposite signs when magnetic field direction change from [111],[110] to [001].It indicated that anomalous Hall Effect can be tune by strength and direction of magnetic field.4)Large magnetoresistance effect in half-Heusler alloys RPt Bi(R=Tb,Dy,Ho,Er and Tm).Weyl semimetal usually hosts a large magnetoresistance due to their band structure with linear dispersion relationship.We found that Lanthanide contraction effect causes the lattice constant decrease with the atomic number of R increasing.Under the geometry of B//[001] and I[100],the magnetoresistance of RPt Bi(R=Tb,Dy,Ho,Er and Tm)show a increasing with the increasing of R's atomic number and the type of curves become more linear.Quantum interference effect in RPt Bi.RPt Bi compounds show a crossover between Weak antilocalization and Weak localization when changing the rear earth R.The analysis indicated this phenomenon origin from the enhancement of spin-dependent scattering.All of results indicate that lattice constant,magnetoresiatance and Quantum interference effect in RPt Bi can be tune by changing rear earth R.