Study On Metal-to-insulator Transition Related Problems In VO₂

Metal-to-insulator?M-I?transition in vanadium dioxide?VO₂?,occurring at a temperature T_c³40 K,has been attracting a lot of attentions due to that the physical nature involving the transition has not yet reached consensus,although many theories or models have been proposed.Accompanying the transition a sharp variation of resistivity of magnitude in several orders occurs,which proposes a positive prospect of application.However the transition temperature of VO₂ is much higher than room-temperature,which limits its application value.Therefore the other hot point is how to realize M-I transition near room-temperature.In this paper,the issues related to M-I transition have been studied based on experimental studies of electrical and magnetic properties,considering a relation of M-I transition and magnetic transition characterized by sudden variation in magnetic susceptibility,magnetic behaviors before and after transition,and doping effects by high-valence cations.The main research works and results are presented as followed:1.Based on experimental studies of temperature-dependent resistivity and magnetic susceptibility of VO₂ samples,it is experimentally revealed that M-I transition and magnetic transition characterized by sudden variation in magnetic susceptibility occur at the same temperature range,showing closely relationship between the two transitions.2.According to apply electron paramagnetic resonance?EPR?technology,which is highly sensitive to magnetic property,to study paramagnetic behavior before and after transition for VO₂ samples.Results show that no observable EPR signal in samples above the transition temperature,and significant EPR signals are observed below the transition temperature and the signal intensities increase with decreasing temperature.Based on analyses to EPR experiment results,physical environment of d electrons from V⁴⁺ions,spin-spin and spin-orbit coupling are analyzed and discussed.3.The temperature dependences of magnetic susceptibilities before and after transition for VO₂ samples are studied.The sample above transition temperature is shown to be Pauli paramagnetism?PM?,while paramagnetic behavior of the sample below transition temperature can't be explained by Pauli PM or Curie PM theory.A phenomenological expression which can quantitatively explain the low-T paramagnetic susceptibility is proposed based on analyses to experimental data.4.According to a theoretical approach of electron states by modeling two-electron system,all kinds of anomalous phenomenon observed in VO₂ can be well explained.It is concluded that no spin exchange exists between d electrons from adjacent V⁴⁺ions on V-chains,as a result,the system shows high-T metallic and Pauli PM behavior,while the anomalous magnetism at low-T is related to spin exchange which exists between d electrons from adjacent V⁴⁺ions on V-chains.Because of the spin exchange, a four-fold degenerate electron state splits into a singlet level of lower energy with S=0?dimer state?and a triplet level of higher energy with S=1.The observed M-I transition and magnetic transition characterized by sudden decrease in magnetic susceptibility is therefore explained to be due to a transition from high-T Pauli PM metallic state to low-T dimerized state with electrons strongly localized.It is further concluded that some unpaired electrons created by thermal activation from singlet to triplet levels contribute to Curie PM and residual V⁴⁺ions which don't participate in the formation of dimers contribute to Pauli PM.An expression that quantitatively explains the temperature-dependent paramagnetic susceptibility at low-T is proposed.Based on fitting to temperature-dependent susceptibilities at high-and low-T,it is thought that⁸5%V⁴⁺ions are paired into dimers by pairing with spin in contrast accompanying transition.5.Nb and Ta doping effects on M-I transition temperature are studied by V_1-xNb_xO₂ and V_1-xTa_xO₂ samples.Results show that the transition temperature decreases at a rate of ^-13 K/at.%with increasing doping level for Nb-doped samples.However,doping causes a significant increase in transition temperature width and an obvious decrease of the magnitude of resistivity abrupt variation.Compared with Nb doping,the rate of decrease of the transition temperature caused by Ta doping is slightly reduced,while Ta doping does not cause significant changes of transition temperature width and resistivity variation magnitude.In 3-4%Ta-doped samples,M-I transition occurs near room-temperature and maintains abrupt variation in two orders of magnitude for resistivity.Magnetic susceptibility measurements indicate that the magnetic phase transition characterized by sudden decrease in magnetic susceptibility and M-I transition occur at nearly the same temperature,showing a significant correlation between the two transitions.6.The magnetic properties of Nb-doped samples before and after the transition are experimentally studied and quantificationally analyzed.Quantitative analyses of the magnetic susceptibilities at high-T show that the doping does not change high-T Pauli PM,and introduces additional electrons into the system.The low-T susceptibility measurements show that the doped samples have stronger temperature dependence than the undoped sample.By considering collaborative contribution to magneticsusceptibility from thermally activated electrons of the dimers,residual V⁴⁺ions, additional introduction electrons and induced V³⁺ions by doping,it is proposed an expression to quantitatively explain temperature-dependent magnetic susceptibility at low-T.The expression fitting results show that Nb doping introduces V³⁺ions into the system in addition to introducing additional electrons.Based on the comprehensive analyses of the experimental results and reported results from literatures,it is concluded that the increase of V-V spacing caused by cations doping of larger size is main point leading to the decrease of M-I transition temperature.