Electronic Transport Properties And Phase Transitions In Several Typical Low-dimensional Confined Systems Under High Pressure

Low-dimensional confined systems exhibit diverse compositional elements,multidimensional structures,rich electronic structures,and electronic transport properties.They serve as crucial platforms for exploring condensed matter physics and find widespread applications in energy storage,biomedical,high-efficiency catalysis,and optoelectronic devices.High pressure,as one of the three thermodynamic parameters alongside temperature and chemical composition,effectively reduces interatomic distances,modulates crystal and electronic band structures,and provides a powerful tool to control various novel properties in low-dimensional confined materials.Under pressure,low-dimensional materials often exhibit novel phenomena,including enhanced superconductivity,altered electronic transport mechanisms,and abundant structural phase transitions.Additionally,high-pressure techniques can further enhance confinement effects,allowing the observation of unique physical and chemical properties not present in bulk materials.This essay focuses on three typical low-dimensional confined systems as research subjects.Using high-pressure in-situ techniques such as electrical transport measurement,X-ray diffraction（XRD）,and Raman spectroscopy,combined with electrical transport model fitting,the study explores the following aspects:1.High-pressure in-situ electrical transport measurements and synchrotron X-ray diffraction techniques were employed to study the electrical transport properties,superconductivity,critical magnetic field,and structural changes of two-dimensional confined materials,Nb₂CS₂ MXene and template Nb₂C.These results reveal a significant enhancement in conductivity of Nb₂C at 46 GPa（400 times higher than conductivity at initial pressure）.At pressure below 20 GPa,2D variable range hopping model fitting shows a decrease in electron localization in Nb₂C due to enhanced interlayer interactions.However,above 20 GPa,structural transition from hexagonal to orthorhombic leads to a change in the electronic transport mechanism,indicating that unconfined MXenes struggle to maintain a stable layered structure under ultra-high pressure.In contrast,S layers in Nb₂CS₂ MXene stabilized structure and improved electrical transport performance.High-pressure electrical transport experiments further indicate that Nb₂CS₂ maintains a metallic-like electrical transport behavior and its confined structure at ultra-high pressure 146 GPa.Meanwhile,Nb₂CS₂ retains robust superconductivity with T_c>8 K up to the maximum applied pressure of 146?GPa.Moreover,the upper critical magnetic field H_c2（0）of Nb₂CS₂ increases with pressure,and the Pauli limit is violated at pressures greater than 120?GPa.Meanwhile,H_c2（0）increases to 19.3?T at 146?GPa,which is 4.8 times greater than at the initial pressure.Further analysis suggests that the significant enhancement of H_c2（0）below 30?GPa comes from the sharp pressure-induced rise of carrier concentration as the interlayer distance decreases,and the significant increase in H_c2（0）above 86?GPa may come from enhanced spin-orbit coupling or the possible unconventional superconducting pairing mechanisms.2.High-pressure in-situ electrical transport measurements and synchrotron XRD were used to investigate the superconducting properties,structural phase transitions,and electronic transport changes in two-dimensional confined material Nb₂CSe₂MXene.The ordered arrangement of Se layers in Nb₂CSe₂ MXene introduced a hybrid confinement structure of TMD+MXene,significantly enhancing its electrical transport and structural stability.At ambient pressure,Nb₂CSe₂ exhibits metallic-like transport behavior but lacks superconductivity.However,at pressure around 23 GPa,a pressure-induced superconducting phase appeared with superconducting transition temperature T_c of 2.2 K.With increasing pressure,T_c and the upper critical magnetic field H_c2increased synchronously(T_c increasing from 2.2 K at 30 GPa to 6.9 K at 175 GPa;H_c2increasing from 4.1 T at 30 GPa to 6.1 T at 153 GPa.).From the High-pressure X-ray diffraction analysis,we find an isostructural phase transition around superconducting transition pressure.The isostructural transition is evidenced by a turning point in cell volume and a sudden change in bulk modulus at 34.1 GPa（B₀ increasing from 25 GPa to 66 GPa）.Above 30 GPa,Nb₂CSe₂transforms from anisotropic compression along the c-axis to isotropic compression.It further indicates that the superconductivity rises from the two-dimensional to three-dimensional structural transition.Under pressure,the enhanced interatomic interactions and bonding between Se atomic layers promotes electron-phonon coupling which further causing superconductivity.Additionally,the ordered arrangement of confined Se atomic layers stabilized the structure and suppressed changes in the Nb₂CSe₂ lattice structure during the phase transition.3.High-pressure in-situ electrical transport measurements,synchrotron XRD,and Raman spectroscopy were employed to investigate the structural and electrical transport property changes of one-dimensional confined system:sulfur chains confined in single-walled carbon nanotubes（S@SWCNT）.Through the characteristic vibrational modes of carbon nanotubes,we find S@SWCNT exhibits slight structural changes at pressure below 3.5 GPa.At higher pressure,S@SWCNT experiences distortion with a more rigid structure than empty tube.High-pressure XRD experiments reveal that two types of sulfur chains（linear chains and zigzag chains）vary differently under pressure.At pressure higher than 15.2 GPa,the linear chains undergo structural distortion and lost long-range order.However,the zigzag chains exhibit higher compressibility and better stability,maintaining an ordered structure even at pressures above 36.7 GPa.From the electrical transport measurements up to 88.9 GPa,Due to the structure distortion,the conductivity and carrier concentration of S@SWCNT decreases monotonically with pressure.At pressure below 15.2 GPa,S@SWCNT exhibits a dual conduction mechanism of metal+2D VRH.Notably,the metal-like electric transport of sulfur chains remain almost unchanged under low-pressure.However,electron localization length of carbon nanotubes keeps decreasing with pressure.At pressure above 60 GPa,due to the distortion of the tube structure,the electrical transport mechanism of S@SWCNT shifts into three-dimensional network,leading to further localization of the electron transport.