טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRibak Amit
SubjectMultiple Ground States of the Transition-Metal
Dichalchogenide TaS2
DepartmentDepartment of Physics
Supervisor Professor Amit Kanigel
Full Thesis textFull thesis text - English Version


Abstract

Low dimensional materials have been a major subject of interest in recent years. In particular, the transitions metal dichalcogenides (TMDs), quasi-2D layered materials with weak van der Waals coupling between layers, received a lot of attention. TMDs exhibit strong electron-electron and electron-phonon interactions that lead to complicated phase diagrams showing a variety of ground states.

                       

            In this work we explore the different ground states of the highly polymorphic transition-metal dichalcogenide TaS2.  In order to perform the research we have used a variety of experimental methods including angle-resolved photoemission spectroscopy, muon-spin relaxation, magnetization measurements and electrical transport measurements. 

                       

            In its 1T polytype TaS2 has an insulating ground state, long believed to be a realization of a Mott insulating state on a triangular lattice. This ground state is expected to exhibit antiferromagnetic ordering.

            We challenge this conjecture, and offer a different ground state - a quantum spin liquid state. This unique ground state reconciles with the absence of magnetic ordering in this material and the insulating behavior at low temperatures. Next, we display an insulator-to-superconductor transition in TaS2, which takes the insulating 1T- TaS2, and turns it into a superconductor with transition temperature of 2.7 K. The tuning parameter for this transition is the three-dimensional coordination of the Ta atom within every layer. This parameter can be tuned by annealing the crystal for several hours at mild temperatures. The result is a different polytype - 4Hb- TaS2, composed of alternate stacking of octahedral and trigonal-prismatic layers. The properties of this superconductor were thoroughly investigated in variety of methods. Remarkably, it was found to break time-reversal symmetry which suggest an unconventional, chiral superconductivity.