Mass enhancement and magnetic order at the Mott-Hubbard transition

Truncated mass divergence in a Mott metal

The Mott metal-insulator transition represents one of the most fundamental phenomena in condensed matter physics. Yet, basic tenets of the canonical Brinkman-Rice picture of Mott localization remain to be tested experimentally by quantum oscillation measurements that directly probe the quasiparticle Fermi surface and effective mass. By extending this technique to high pressure, we have examined the metallic state on the threshold of Mott localization in clean, undoped crystals of NiS2. We find that i) on approaching Mott localization, the quasiparticle mass is strongly enhanced, whereas the Fermi surface remains essentially unchanged; ii) the quasiparticle mass closely follows the divergent form predicted theoretically, establishing charge carrier slowdown as the driver for the metal-insulator transition; iii) this mass divergence is truncated by the metal-insulator transition, placing the Mott critical point inside the insulating section of the phase diagram. The inaccessibility of the Mott critical point in NiS2 parallels findings at the threshold of ferromagnetism in clean metallic systems, in which criticality at low temperature is almost universally interrupted by first-order transitions or novel emergent phases such as incommensurate magnetic order or unconventional superconductivity.

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)2Cu[N(CN)2]Br and κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, performed in a wide temperature range is reported. The latter two compounds were located near the metal–insulator boundary in the dimer-Mott phase diagram. From the comparison of the temperature dependences of their heat capacities, we indicated that lattice heat capacities of the partially deuterated compounds were larger than that of the pristine compound below about 40 K. This feature probably related to the lattice softening was discussed also by the sound velocity measurement, in which the dip-like structures of the Δv/v were observed. We also discussed the variation of the electronic heat capacity under magnetic fields. From the heat capacity data at magnetic fields up to 6 T, we evaluated that the normal-state γ value of the partially deuterated compound, κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, was about 3.1 mJ K−2 mol−1. Under the magnetic fields higher than 3.0 T, we observed that the magnetic-field insulating state was induced due to the instability of the mid-gap electronic state peculiar for the two-dimensional dimer-Mott system. Even though the volume fraction was much reduced, the heat capacity of κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br showed a small hump structure probably related to the strong coupling feature of the superconductivity near the boundary.

Mott Metal-Insulator Transitions in Pressurized Layered Trichalcogenides

Transition metal phosphorous trichalcogenides, MPX_{3} (M and X being transition metal and chalcogen elements, respectively), have been the focus of substantial interest recently because they are unusual candidates undergoing Mott transition in the two-dimensional limit. Here we investigate material properties of the compounds with M=Mn and Ni employing ab initio density functional and dynamical mean-field calculations, especially their electronic behavior under external pressure in the paramagnetic phase. Mott metal-insulator transitions (MIT) are found to be a common feature for both compounds, but their lattice structures show drastically different behaviors depending on the relevant orbital degrees of freedom, i.e., t_{2g} or e_{g}. Under pressure, MnPS_{3} can undergo an isosymmetric structural transition within monoclinic space group by forming Mn-Mn dimers due to the strong direct overlap between the neighboring t_{2g} orbitals, accompanied by a significant volume collapse and a spin-state transition. In contrast, NiPS_{3} and NiPSe_{3}, with their active e_{g} orbital degrees of freedom, do not show a structural change at the MIT pressure or deep in the metallic phase within the monoclinic symmetry. Hence NiPS_{3} and NiPSe_{3} become rare examples of materials hosting electronic bandwidth-controlled Mott MITs, thus showing promise for ultrafast resistivity switching behavior.

Spectroscopic Studies on the Metal-Insulator Transition Mechanism in Correlated Materials

The metal-insulator transition (MIT) in correlated materials is a novel phenomenon that accompanies a large change in resistivity, often many orders of magnitude. It is important in its own right but its switching behavior in resistivity can be useful for device applications. From the material physics point of view, the starting point of the research on the MIT should be to understand the microscopic mechanism. Here, an overview of recent efforts to unravel the microscopic mechanisms for various types of MITs in correlated materials is provided. Research has focused on transition metal oxides (TMOs), but transition metal chalcogenides have also been studied. Along the way, a new class of MIT materials is discovered, the so-called relativistic Mott insulators in 5d TMOs. Distortions in the MO₆ (M = transition metal) octahedron are found to have a large and peculiar effect on the band structure in an orbital dependent way, possibly paving a way to the orbital selective Mott transition. In the final section, the character of the materials suitable for applications is summarized, followed by a brief discussion of some of the efforts to control MITs in correlated materials, including a dynamical approach using light.

Spin ordering and enhancement of electronic heat capacity in an organic system of (DI-DCNQI)(2)(Ag(1-x)Cu(x))

Thermodynamic measurements on the organic system of (DI-DCNQI)(2)(Ag(1-x)Cu(x)) (x = 0,0.05, 0.71, 0.90) were performed to study the change from the charge-ordered (CO) insulating state to the π-d hybridized metallic state. A thermal anomaly associated with the antiferromagnetic transition that occurred in the charge-ordered lattice was observed at 6.2 K from the temperature dependence of the heat capacity of (DI-DCNQI)(2)Ag. We have found that the magnetic entropy around the peak is only 1.5% of Rln2, corresponding to the full entropy expected for the formula unit of (DI-DCNQI)(2)Ag. This anomaly is suppressed down to about 3 K in the x = 0.05 sample owing to the disorders induced in the CO lattice. In the metallic concentration of x = 0.90, the low-temperature electronic heat capacity coefficient, γ was found to be enhanced by up to about 63.6 mJ K(-2) mol(-1) probably owing to the cooperative effect of π-d hybridization and intersite Coulomb interaction (V).

Electronic and local structural changes in Li2+xTi3O7ramsdellite compounds upon electrochemical Li-ion insertion reactions by X-ray absorption spectroscopy

Electronic and local structural changes in ramsdellite-type Li(2+x)Ti3O7 compound were investigated by X-ray absorption spectroscopy (XAS) measurements. Upon electrochemical Li-ion insertions, the host lattice with ramsdellite structure is retained, indicated by X-ray powder diffraction. Ti K-edge extended X-ray absorption fine structure (EXAFS) analysis shows, however, slight local structural distortions around Ti ions. The energy shifts and the changes in the peak intensity of Ti K-edge and Ti L-edge XAS reveal the reducing oxidation states of Ti ions as the amount of electrochemically-inserted Li-ion increases. Equally important, oxide ions have a significant effect on the electronic transfer process, suggested by O K-edge XAS. These results on electronic structural changes were interpreted using the Zaanen-Sawatzky-Allen scheme.

Critical behavior of metal-insulator transition in La1-(x)Sr(x)VO3

Critical behavior of the metal-insulator transition (MIT) coupled with spin/orbital correlations has been investigated for single crystals of La1-(x)Sr(x)VO3. In the paramagnetic (PM) metal phase (x > 0.260), the precursor to the MIT manifests itself as an enhancement of carrier effective mass. In the antiferromagnetic (AF) metal phase (0.178 < or = x < or = 0.260), the carrier density decreases and the correlation of the orbital seems to evolve towards the MIT (x = 0.178). In the AF insulating phase (x < 0.178), the distinct first-order structural phase transition occurs with the decrease of temperature, perhaps concomitantly with the orbital ordering.

Dynamical Signature of the Mott-Hubbard Transition in Ni(S,Se)2

The transition metal chalcogenide Ni(S,Se)2 is one of the few highly correlated, Mott-Hubbard systems without a strong first-order structural distortion that normally cuts off the critical behavior at the metal-insulator transition. The zero-temperature (T) transition was tuned with pressure, and significant deviations were found near the quantum critical point from the usual T1/2 behavior of the conductivity characteristic of electron-electron interactions in the presence of disorder. The transport data for pressure and temperature below 1 kelvin could be collapsed onto a universal scaling curve.