Nuclear Magnetic Resonance Partial Application In The Physical Study Of Solid Materials

Nuclear magnetic resonance(NMR)is a cutting-edge scientific method,which plays an important role in scientific research and daily life.Since invented by Purcell and Bloch in 1945,NMR has evolved into a very important comprehensive discipline.As a means of analyzing materials,NMR can quickly and accurately detect structural information of the material without damage,and can detect information even at atomic level.Although it originated from physics,NMR has expanded into the fields of chemistry,biology,geology,medicine and so on.Currently,NMR research focuses more on chemistry and biology than on physics.There are few reports on solid state NMR in material science especially in China.This field is still in the preliminary stage.As an efficient means of research,NMR can quickly and accurately test crystal structure,analyze electronic structure,track various phases and so on.This makes it have great applications in the field of physics.Based on the application of NMR in several hot issues,this thesis shows the significance of NMR in physical research.We hope our work could promote the domestic development in this field,and attract more researchers to join in.The main points of this thesis are as follows:NMR measurement of energy gap.For semiconductors,the energy gap(between the top of the valence band and the bottom of the conduction band in semiconductor or insulator)is one of the most fundamental and important parameters.The main methods of measuring energy gap include light absorption spectrometry,electrical resistance technique,and Seebeck coefficient method,etc.However,most of these methods have demanding requirements for the size of samples.These methods are difficult to operate for powder samples.NMR does not require for sample size.Bulk or powder and even nanometer scale sample can be measured by NMR.We measured the samples of black phosphorus in powder form.The energy gap can be obtained through the fitting of the relationship between spin-lattice rates and temperature.At the same time,the energy gap can be obtained with the method of measuring the Seebeck coefficients as a function of temperatures,too.The gap values obtained by the two different methods are well matched,which proves the accuracy of the NMR gap measurement.NMR identifying and tracking phases.The basic principle of NMR is the resonance measurement of nuclear spin's precession frequency in steady magnetic field.Different atoms have different resonant frequencies,which can be used to identify the elements.More importantly,the nucleus in a solid is surrounded by electron cloud.The electron cloud will bring magnetic shielding effect,which makes the magnetic field nuclear received have some difference and lead to resonance frequency deviation.This deviation is called shift,which carries the electron structure and crystal structure information around the atom.So it can be used to characterize phases.The oxidization problem of black phosphorus has been considered as an obstacle to further research and application.Through the real-time nuclear magnetic resonance measurement of black phosphorus,the changes in the content of black phosphorus and the generation new phase were tracked dynamically.The black phosphorus oxidation process was explored systematically,which is a breakthrough for preventing the oxidation of black phosphorus.In addition,we have tracked and observed the various phases of doped Te and hydrogen absorbed black phosphorus,and verified the effectiveness of these two methods to prevent the oxidation of black phosphorus.NMR characterization of electron spin-orbit coupling.As a hot field in recent years,topological insulators have been widely studied.It is an important point to seek new materials with topological insulator properties.This is mainly measured by angular resolution photoelectron spectroscopy.However,the angular resolution photoelectron spectroscopy is not applicable to some materials and very expensive.It is worth noting that the strong spin-orbit coupling is a necessary condition for topological insulators.So it is possible to preliminarily screen materials by measuring the coupling strength of spin-orbit.The shift of NMR reflects electronic structure,which is possible to measure the coupling strength of spin-orbit.We prepared a series of half Heusler alloys.Their spin-orbit coupling was calculated and their shift were measured.We found the corresponding relationship between them.This would laid the foundation for the development of new technologies.NMR manifestation of strong electron correlation.Strong electron correlation system has many strange physical properties,such as high temperature superconductivity,fractional quantum hall effect in two-dimensional electron gas,giant magnetoresistance effect of manganese oxide materials and heavy Fermi subsystem,two-dimensional high mobility of material with metal-insulator phase transition,quantum phase transition and critical phenomena,charge density wave in one-dimensional conductors,etc.All these features have significant research value and application prospects.For quadruple nuclei in non-zero electric field gradients,the peak of its nuclear magnetic resonance spectrum will be split,resulting in multiple satellite lines.These satellites contain electric field gradient information,which can be used to characterize the electric field gradients.At the same time,the electric field gradients are determined by both ion cores and valence electrons.If there is an abnormal change in the electric field gradients,there may be a strong correlation.We studied VO2 which was claimed to be a strong electron system and found an anomalous change in temperature dependence of the electric field gradients in the high temperature phase.Combined with other measurements,we prove that there exists Mott orbital selection in VO2 which is the signature of strong electron correlation.This will help to further reveal the mechanism of metal insulator phase transition and understand high temperature phase strange physical phenomena in VO2 mechanism.