Structure-dependent metal insulator transition in one-dimensional Hubbard superlattice

We investigate the charge and spin gaps, and the spin structure in half-filled one-dimensional Hubbard superlattices with one repulsive site and L0 free sites per unit cell. For odd L0, it is correlated metal at the particle–hole symmetric point, and then turns into band insulator beyond this point. For even L0, the system has a Mott insulator phase around the particle–hole symmetric point and undergoes a metal–insulator transition with on-site repulsion U increasing. For large U,there exists a multiperiodic spin structure, which results from the ferromagnetic（antiferromagnetic） correlation between the nearest neighboring repulsive sites for odd（even） L0.

Quantitative determination of the critical points of Mott metal–insulator transition in strongly correlated systems

Mottness is at the heart of the essential physics in a strongly correlated system as many novel quantum phenomena occur in the metallic phase near the Mott metal–insulator transition. We investigate the Mott transition in a Hubbard model by using the dynamical mean-field theory and introduce the local quantum state fidelity to depict the Mott metal–insulator transition. The local quantum state fidelity provides a convenient approach to determining the critical point of the Mott transition. Additionally, it presents a consistent description of the two distinct forms of the Mott transition points.

Pressure induced insulator to metal transition in quantum spin liquid candidate NaYbS_(2)

Pressure induced insulator to metal transition followed by the appearance of superconductivity has been observed recently in inorganic quantum spin liquid candidate NaYbSe_(2).In this paper,we study the properties of isostructural compound NaYbS_(2)under pressure.It is found that the resistance of Na YbS_(2)single crystal exhibits an insulating state below 82.9 GPa,but with a drop of more than six orders of magnitude at room temperature.Then a minimum of resistance is observed at about 100.1 GPa and it moves to lower temperature with further compression.Finally,a metallic state in the whole temperature range is observed at about 130.3 GPa accompanied by a non-Fermi liquid behavior below 100 K.The insulator to metal transition,non-monotonic resistance feature and non-Fermi liquid behavior of NaYbS_(2)under pressure are similar to those of NaYbSe_(2),suggesting that these phenomena might be the universal properties in NaLnCh_(2)(Ln=rare earth,Ch=O,S,Se)system.

Rubidium-induced phase transitions among metallic,band-insulating,Mott-insulating phases in 1T-TaS_(2)

Realizing phase transitions via non-thermal sample manipulations is important not only for applications,but also for uncovering the underlying physics.Here,we report on the discovery of two distinct metal–insulator transitions in 1T-TaS_(2) via angle-resolved photoemission spectroscopy and in-situ rubidium deposition.At 205 K,the rubidium deposition drives a normal metal–insulator transition via filling electrons into the conduction band.While at 225 K,however,the rubidium deposition drives a bandwidth-controlled Mott transition as characterized by a rapid collapsing of Mott gap and a loss of spectral weight of the lower Hubbard band.Our result,from a doping-controlled perspective,succeeds in distinguishing the metallic,band-insulating,and Mott-insulating phases of 1T-TaS_(2),manifesting a delicate balance among the electronitineracy,interlayer-coupling and Coulomb repulsion.We also establish an effective method to tune the balance between these interactions,which is useful in seeking exotic electronic phases and designing functional phase-changing devices.

Doping dependent metal to insulator transition in the(Bi,Pb)-2212 system:The evolution of structural and electronic properties with europium substitution

The present work investigates the effect of europium substitution on the （Bi, Pb）-2212 system in the concentration range 0.5 ≤ x ≤1.0. Phase analysis and lattice parameter calculations on the powder diffraction data and the elemental analysis of EDX show that the Eu atoms are successfully substituted into the （Bi, Pb）-2212 system. Resistivity measurements （64-300 K） reveal that the system exhibits superconductivity at x ≤ 0.5 and semiconductivity at x 〉 0.5. With the complete suppression of superconductivity which is known to be a quasi-two dimensional phenomenon in these materials, a metal to insulator transition takes place at x = 0.6 and the predominant conduction mechanism is found to be variable range hopping between localized states, resulting in macroscopic semiconducting behaviour. The results of electrical and structural properties of the doped （Bi, Pb）-2212 compounds suggest that the decrease of charge carrier concentration and the induced structural disorder are the more effective and dominant mechanisms in the origin of the metal to insulator transition and suppression of superconductivity due to Eu substitution at its Sr site.

In situ electronic structural study of VO_2 thin film across the metal–insulator transition

The in situ valence band photoemission spectrum （PES） and X-ray absorption spectrum （XAS） at V LⅡ-LⅢ edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal–insulator transition （MIT） temperature （TMIT=67 ℃）. The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LⅢ-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator–semiconductor, semiconductor–metal processes, and vice versa. The conventional MIT at around the TMIT=67 ℃ is actually a semiconductor–insulator transformation point.

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO2-terminated BaTiO3（001） surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3（001） surface.

Transforming a Two-Dimensional Layered Insulator into a Semiconductor or a Highly Conductive Metal through Transition Metal Ion Intercalation

Atomically thin two-dimensional(2D) materials are the building bricks for next-generation electronics and optoelectronics, which demand plentiful functional properties in mechanics, transport, magnetism and photoresponse.For electronic devices, not only metals and high-performance semiconductors but also insulators and dielectric materials are highly desirable. Layered structures composed of 2D materials of different properties can be delicately designed as various useful heterojunction or homojunction devices, in which the designs on the same material(namely homojunction) are of special interest because preparation techniques can be greatly simplified and atomically seamless interfaces can be achieved. We demonstrate that the insulating pristine ZnPS_3, a ternary transition-metal phosphorus trichalcogenide, can be transformed into a highly conductive metal and an n-type semiconductor by intercalating Co and Cu atoms, respectively. The field-effect-transistor(FET) devices are prepared via an ultraviolet exposure lithography technique. The Co-ZnPS_3 device exhibits an electrical conductivity of 8 × 10^(4) S/m, which is comparable to the conductivity of graphene. The Cu-ZnPS_3 FET reveals a current ON/OFF ratio of 1-05 and a mobility of 3 × 10^(-2 )cm^(2)·V^(-1)·s^(-1). The realization of an insulator, a typical semiconductor and a metallic state in the same 2D material provides an opportunity to fabricate n-metal homojunctions and other in-plane electronic functional devices.

Manipulation of band gap in 1T-TiSe_(2) via rubidium deposition

The 1T-TiSe_(2) is a two-dimensional charge-density-wave(CDW)material that attracts great interest.A small band gap locates at the Fermi level separating the Ti d-bands and Se p-bands,which makes 1T-TiSe_(2) a promising candidate for realizing excitonic condensation.Here,we studied the band gap in 1T-TiSe_(2) using angle-resolved photoemission spectroscopy(ARPES).Instead of only focusing on the in-plane band dispersions,we obtained the detailed band dispersions of both conduction and valance bands along the out-of-plane direction.We found that the conduction and valance bands split into multiple sub-bands in the CDW state due to band folding.As a result,the band gap between the Ti d-bands and Se p-bands reduces to~25 meV and becomes a direct gap in the CDW state.More intriguingly,such band gap can be further reduced by the rubidium deposition.The band structure becomes semimetallic in the rubidium-doped sample.Meanwhile,exotic gapless behaviors were observed at the p-d band crossing.Our result characterized the band gap of 1T-TiSe_(2) in three-dimensional Brillouin zone with unpreceded precision.It also suggests a closing of band gap or a potential band inversion in 1T-TiSe_(2) driven by rubidium deposition.

Surface doping manipulation of the insulating ground states in Ta_(2)Pd_(3)Te_(5) and Ta_(2)Ni_(3)Te_(5)

Manipulating emergent quantum phenomena is a key issue for understanding the underlying physics and contributing to possible applications.Here we study the evolution of insulating ground states of Ta_(2)Pu_(3)Te_(5) and Ta_(2)Ni_(3)Te_(5) under in-situ surface potassium deposition via angle-resolved photoemission spectroscopy.Our results confirm the excitonic insulator character of Ta_(2)d_(3)Te_(5).Upon surface doping,the size of its global gap decreases obviously.After a deposition time of more than 7 min,the potassium atoms induce a metal-insulator phase transition and make the system recover to a normal state.In contrast,our results show that the isostructural compound Ta_(2)Ni_(3)Te_(5) is a conventional insulator.The size of its global gap decreases upon surface doping,but persists positive throughout the doping process.Our results not only confirm the excitonic origin of the band gap in Ta_(2)Pd_(3)Te_(5),but also offer an effective method for designing functional quantum devices in the future.

Frequency regulation in alternation-current transports across metal to insulator transitions of thin film correlated perovskite nickelates

Although the metal to insulator transition(MIT)observed in d-band correlated metal oxides enables promising applications(e.g.,correlated logical devices and Mottronic devices),its present recognition is mainly limited on the direct current(DC)electrical transports.Up to date,the MIT from the perspective of alternation current(AC)transport and its potential electronic applications remains yet unclear.Herein,we demonstrate the frequency(f_(AC))dependence in the impedance(Z=Z’+iZ″)of typical MIT materials,such as thin film rare-earth nickelates(Re NiO_(3)),across the critical MIT temperature(T_(MIT)).Apart from the abrupt change in the impedance modulus(|Z|)across the critical temperature(T_(MIT))similar to the DC transport,the MIT also triggers non-continuous variation in the impedance phase(θ),and this enables the f_(AC)-regulations in the Z’-T tendencies(Z’=|Z|cosθ).At the critical f_(AC) range(e.g.,104-106 Hz),the con-versing variations in|Z|-T and cosθ-T across T_(MIT) result in non-monotonic delta-shape Z’-T tendency in Sm_(x) Nd_(1-x) NiO_(3),the full width half maximum of which is effectively narrowed compared to the situation with the absence of MIT.Further imparting lower or higher f_(AC) elevate the domination in|Z|-T and cosθ-T,respectively,but also enables abrupt Z’-T tendencies across T_(MIT) showing negative temperature coefficient of resistance(NTCR)or positive temperature coefficient of resistance(PTCR).By introducing f_(AC) as a new freedom,the MIT behavior can be more comprehensively regulated electronically,and this extends the vision in exploring the new electronic applications based on the correlated MIT materials from the AC perspective.

Defect energetics and magnetic properties of 3d-transition-metal-doped topological crystalline insulator SnTe

The introduction of magnetism in SnTe-class topological crystalline insulators is a challenging subject with great importance in the quantum device applications. Based on the first-principles calculations, we have studied the defect energetics and magnetic properties of 3d transition-metal(TM)-doped SnTe. We find that the doped TM atoms prefer to stay in the neutral states and have comparatively high formation energies, suggesting that the uniform TMdoping in SnTe with a higher concentration will be difficult unless clustering. In the dilute doping regime, all the magnetic TMatoms are in the high-spin states, indicating that the spin splitting energy of 3d TM is stronger than the crystal splitting energy of the SnTe ligand. Importantly, Mn-doped SnTe has relatively low defect formation energy, largest local magnetic moment, and no defect levels in the bulk gap, suggesting that Mn is a promising magnetic dopant to realize the magnetic order for the theoretically-proposed large-Chern-number quantum anomalous Hall effect(QAHE) in SnTe.

Magnetic Properties and Colossal Magnetoresistance of La_(0.7)Sr_(0.3)MnO_3 Materials Doped with Fe

La 0.7 Sr 0.3 Fe x Mn 1- x O 3 compounds (0 05≤ x ≤0 18) were prepared by the sol gel technique. The effects of Fe doping on the magnetic properties, conductivity and magnetoresistance for La 0.7 Sr 0.3 MnO 3 was investigated. Experimental results indicate that the Fe doping leads to a decrease in the ferromagnetic ordering temperature, an increase in the resistance and magnetoresistance effect. For samples of x =0 05 and x =0 08 two peaks are observed in their ρ T curve. Magnetic resistivity ( MR ) of La 0.7 Sr 0.3 Fe 0.08 Mn 0.92 O 3 sample is capable of reaching 18%(8×10 5A·m -1 , 293 K). At x ≥0 1, metal insulator (M I) transition temperature ( T p) is lower than ferromagnetic Curie temperature ( T C) by about 50～80 K.

Effects of A-Cation Mismatch σ~2 on Transport Properties of Manganese Doped Oxide Perovskites

Three factors control the transition from paramagnetic insulator to ferromagnetic metal. The first is the hole doping. The second factor is the average ionic radius of the A site cation rA. The last one concerns the ionic size mismatch σ^2 at the A site. In order to study the effect of σ^2, a series of samples were prepared with constant value of x and rA.

Correlated perovskite nickelates with valence variable rare-earth compositions

While the metal to insulator transition(MIT)of d-band correlated perovskite nickelates(RENiO_(3))are widely adjustable via their rare-earth composition,the roles of potential valence variabilities associated with the rare-earth elements were rarely concerned.Herein,we demonstrate the material synthesis and MIT properties of RENiO_(3) containing valence variable rare-earth compositions,such as Ce,Pr,Sm,Eu and Tb.The metastable perovskite structure of SmNiO_(3) and EuNiO_(3) with a rare-earth valence states variable towards+2 can be effectively synthesized under high oxygen pressures as it is necessary to reduce their formation free energies.This is in contrast to Ce and Tb,in which situations the variable rare-earth valence state towards+4 reduces their ionic radius and prohibits their occupation or co-occupation of the rare-earth site within the perovskite structured RENiO_(3).Nevertheless,PrNiO_(3) with MIT properties can be effectively synthesized at lower oxygen pressures,owing to the higher stability to form a fully occupied 6s orbit associated Pr3+compared to the half-filled one related to Pr4+.The present work provides guidance for regulating the MIT properties of RENiO_(3).

Electronic and magnetic stability in correlated transportations of rare-earth nickelate perovskites

Although the thermistor and metal to insulator transition bi-functionalities were discovered for rareearth nickelates(RENiO3),the electronic stability in their correlated transports under impulse voltage or magnetic field remain as open questions.Herein,we demonstrate the thermistor transportations of the electron correlated rare-earth nickelates under impulse direct current voltage and in magnetic environment.The insulating phase of RENiO3 shows zero crossing linear I-V characters,indicating their stable electronic resistance is independent of the imparted voltage up to 10 V and pulse width down to1 us,in spite of their sensitive electronic structures to polarizations.In addition,the high electronic stability associated with the thermistor transportation of RENiO3 is also demonstrated in magnetic fields up to 9 T(i.e.,MR<0.2%).The high electronic stability further paves the way to applying RENiO3 as a broad temperature range thermistor in temperature sensing or circuit protections for correlated electronics.