Pressure-induced transition from localized electron toward band antiferromagnetism in LaMnO(3)

The temperature dependence of the ac susceptibility under pressure has been used to track the Néel temperature T(N) of the Mott insulators LaMnO3, CaMnO3, and YCrO3. Bloch's rule relating T(N) to volume V, viz., alpha=dlog(T(N)/dlog(V=-3.3, is obeyed in YCrO3 and CaMnO3; it fails in LaMnO3. This breakdown is interpreted to be due to a sharp increase in the factor [U(-1)+(2Delta)(-1)] entering the superexchange perturbation formula. A first-order change at 7 kbar indicates that the transition from localized-electron to band magnetism is not smooth.     

Pressure-induced quenching of the Jahn-Teller distortion and insulator-to-metal transition in LaMnO(3)

LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.     

Nb掺杂导致LaMnO_3绝缘体-金属转变的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算,系统研究了Nb掺杂LaMn_(1-x)Nb_xO_3(x=0,0. 25,0. 5,0. 75)的结构和电磁性质.计算结果表明,所有的LaMn_(1-x)Nb_xO_3都稳定在正交结构. LaMn_(1-x)Nb_xO_3当x 0. 5时为A型反铁磁绝缘体,在x=0. 5和0. 75时为G型反铁磁金属.随着Nb掺杂量增加,当x=0. 5时掺杂电子占据导带的底部,系统产生绝缘体-金属转变.这意味着LaMn_(1-x)Nb_xO_3在电子器件上可能有重要的应用.另外,LaMn_(1-x)Nb_xO_3在x=0. 25和0. 75时出现了自旋玻璃行为.     

Pressure-induced metal-insulator transition in LaMnO3 is not of Mott-Hubbard type

Calculations employing the local density approximation combined with static and dynamical mean field theories (LDA+U and LDA+DMFT) indicate that the metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room temperature is not a Mott-Hubbard transition, but is caused by orbital splitting of the majority-spin eg bands. For LaMnO3 to be insulating at pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller distortion are needed.     

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

The electronic properties of TiO_2-terminated BaTiO_3(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 BaTiO_3(001) surface.     

Vapor-liquid (insulator-metal) phase transition in alkali metal vapors

A simple physical model is proposed that describes a vapor-liquid phase transition in alkali metal vapors. The model is based on an assumption made on the character of binding between atoms in the gas phase near the critical point. This is the collective quantum cohesive energy, well-known in the theory of liquid alkali metals, which arises due to the appearance of conduction electrons and is extended to the gas region near the critical point. The parameters of the critical points of the transition and of the binodal are determined on the basis of the model calculation of the binding energy for all alkali metals. Combined, these parameters well agree with experimental results and the predictions made by other authors. The minimum metallic conductivity is evaluated. Its behavior allows one to conclude that vapor-liquid and insulator-metal transitions in alkali metal vapors coincide. This fact sheds light on the Zel’dovich-Landau problem as applied to alkali metal vapors.     

Pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 investigated by infrared microspectroscopy

We investigated pressure-induced insulator-metal transition in Ca₂Ru_0.92Fe_0.08O₄ by using infrared microspectroscopy. As the pressure is increased up to 1.7 GPa, we observed a large increment of the reflectivity in the entire mid-infrared range. Accompanied by such a clear signature of the insulator-metal transition, we found an evidence of the structural transition from the frequency shift of the Ru-O stretch phonon mode which is attributed to the shortening of the in-plane Ru-O bond length. When we compared these pressure-dependent changes with the corresponding temperature-dependent results, we found that the pressure-induced metallic state has a higher reflectivity as well as the higher phonon frequency. Indeed, it turns out that the pressure-induced metallic state of Ca₂Ru_0.92Fe_0.08O₄ looks very similar with the metallic state of Sr-substituted Ca₂RuO₄ not only in the reflectivity level but also in the phonon frequency. This suggests that the electronic properties are closely related to the structural degree of freedom, and the pressure can be a useful parameter to induce the transitions from the Mott-insulator to the metal and further to the superconductor as observed for Sr₂RuO₄.     

Strontium ferrates(IV): transition metal oxides at the insulator-metal borderline

Oxoferrates with iron in the high formal oxidation state of 4+ show a variety of electronic properties including insulating behavior, metallic conductivity, and a valence disproportionation of Fe⁴⁺. Here, we report investigations of Sr₂FeO₄ which crystallizes in the two-dimensional K₂NiF₄-type structure. From resistivity and magnetic susceptibility measurements it is found that Sr₂FeO₄ is an antiferromagnetic semiconductor with a Néel temperatureT _N of about 60 K. The Mössbauer spectra of the paramagnetic phase of Sr₂FeO₄ reveal a single Fe⁴⁺ quadrupole doublet. Those of the ordered phase consist of a complicated magnetic hyperfine pattern with at least four inequivalent Fe⁴⁺ sites. These may arise from structural distortions and/or from a complicated spin structure. External pressures above 6 GPa lead to an increase in near-infrared oscillator strength indicating a gap-narrowing and possibly an insulatormetal transition. Raman spectra between 20 and 300 K show, in addition to the normal Raman-active phonon modes of the K₂NiF₄-type crystal structure, a further oxygen-derived phonon mode. The additional Raman band vanishes near 5.5 GPa. The electronic behavior of strontium ferrates(TV) is discussed within the general systematics for the electronic structure of transition metal compounds.On leave from: Institute of Crystallography, Russian Academy of Science, Moscow, Russia.     

Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La(1-x)Ba(x)CoO3

The nature of the magnetic ground state near the insulator-metal transition (IMT) in La(1-x)Ba(x)CoO3 was investigated via neutron scattering. Below the critical concentration, x(c)～0.22, a commensurate antiferromagnetic (AFM) phase appears initially. Upon approaching x(c), the AFM component weakens and a ferromagnetic (FM) ordered phase sets in while in the rhombohedral lattice. At x(c), a spin flip to a new FM structure occurs at the same time as the crystal symmetry transforms to orthorhombic (Pnma). The Pnma phase may be the driving force for the IMT.     

Pressure-induced phase transition in β-eucryptite (LiAlSiO4)

A pressure-induced phase transition was found in β-eucryptite using single crystal X-ray and Raman scattering measurements. The high pressure phase forms at a pressure of ～8 kbar at 300 K, is metastable at 1 bar and will revert to the normal low pressure phase with time at this temperature.     

The insulator-metal transition in supported clusters

The size-dependent insulator-metal transition in supported metal clusters manifests itself as a deviation from inverse radius dependence of the core-electron binding energy. Data for mono- and polydisperse supported gold clusters give evidence that the transition to the metallic state occurs in clusters containing circa 100 atoms. Simple theoretical considerations account for this observation.     

Pressure-induced transition in BP

The electrical resitance of BP was measured under ultrahigh static pressures. A cusp was observed around 400 kbar in the resistance vs pressure curve, suggesting the presence of a transition to a nonmetallic state.     

Metal-insulator transition and its relation to magnetic structure in (LaMnO3)2n/(SrMnO3)n superlattices

Superlattices of (LaMnO3){2n}/(SrMnO3){n} (1or=3. Measurements of transport, magnetization, and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in LaMnO3 and suppressed in SrMnO3. The modulation is consistent with a Mott transition driven by the proximity between the (LaMnO3)/(SrMnO3) interfaces. The insulating state for n>or=3 obeys variable range hopping at low temperatures. We suggest that this is due to states at the Fermi level that emerge at the (LaMnO3)/(SrMnO3) interfaces and are localized by disorder.     

Pressure-induced quantum phase transition in the spin-liquid TlCuCl3

The condensation of magnetic quasiparticles into the nonmagnetic ground state has been used to explain novel magnetic ordering phenomena observed in quantum spin systems. We present neutron scattering results across the pressure-induced quantum phase transition and for the novel ordered phase of the magnetic insulator TlCuCl3, which are consistent with the theoretically predicted two degenerate gapless Goldstone modes, similar to the low-energy spin excitations in the field-induced case. These novel experimental findings complete the field-induced Bose-Einstein condensate picture and support the recently proposed field-pressure phase diagram common for quantum spin systems with an energy gap of singlet-triplet nature.     

Pressure-induced phase transition in transition metal trifluorides

As a fundamental thermodynamic variable, pressure can alter the bonding patterns and drive phase transitions leading to the creation of new high-pressure phases with exotic properties that are inaccessible at ambient pressure. Using the swarm intelligence structural prediction method, the phase transition of TiF₃, from R—3c to the Pnma phase, was predicted at high pressure, accompanied by the destruction of TiF₆ octahedra and formation of TiF₈ square antiprismatic units. The Pnma phase of TiF₃, formed using the laser-heated diamond-anvil-cell technique was confirmed via high-pressure x-ray diffraction experiments. Furthermore, the in situ electrical measurements indicate that the newly found Pnma phase has a semiconducting character, which is also consistent with the electronic band structure calculations. Finally, it was shown that this pressure-induced phase transition is a general phenomenon in ScF₃, VF₃, CrF₃, and MnF₃, offering valuable insights into the high-pressure phases of transition metal trifluorides.     

Pressure-induced magnetic transition in MnRhAs

The magnetic properties of a Fe₂P-type intermetallic compound MnRhAs have been investigated under high pressure up to 8.0 GPa by AC susceptibility measurement. Initially, both the antiferromagnetic (AF(I)) to the canted state magnetic transition temperature T_t and the canted state to another antiferromagnetic one (AF(II)) transition temperature T_C increase with compression. At 4.0 GPa, however, T_t decreases abruptly, while the increasing rate of T_C becomes larger above this pressure. A pressure-induced magnetic phase transition was seen at around this pressure when T_t and T_C are plotted in the pressure–temperature phase diagram. The transition from the antiferromagnetic to the ferromagnetic state observed below 160 K with increasing pressure is not frequently observed.