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

Abstract

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.

Filamentary microstructure and linear temperature dependence of normal state transport in optimized high temperature superconductors

“…Moreover even though there is no algorithm that can be used to compute in full detail such phase-coherent ''random'' states explicitly, their number is still countable. These existence and countability arguments have proved to be of great value in recent analyses of the characteristic exponents for the MIT as obtained in three extremely informative experiments on Si:P (17), neutron transmutation doped Ge:Ga [both uncompensated (18) and compensated (19)] and antiferromagnetic Ni(S, Se) 2 alloys (20). The details of the comparison between experiment and theory are lengthy (13-16) and will not be given here, but two important general points can be mentioned.…”

Nonlinear Electric Field Effects at a Continuous Mott-Hubbard Transition

“…Combining the results at all three pressures yields a best fit value z͑͞z 1 1͒ 0.73 10.02 20.03 , so that z 2.7 10.3 20.4 . If one assumes that the conductivity and spatial correlation length exponents are identical, m n ϳ 1, then the previously ascertained dynamical response for NiS 22x Se x [5] would constrain z to lie between 4 and 5. The present analysis of electric field scaling at the pressure-tuned metal-insulator transition establishes directly that the dynamical critical exponent z has a value between 2 and 3, with a best fit value more than 4 standard deviations below z 4.…”

“…In the context of finite-size scaling about a quantum critical point, this trend can be understood as a crossover from scaling controlled by temperature [5] to scaling controlled by the electric field. It is qualitatively consistent as well with IR heating of the electrons.…”

“…An exception to this rule is the transition-metal chalcogenide NiS 22x Se x , a Mott-Hubbard system [4] without a symmetry change at an effectively continuous T 0 metal-insulator transition [5]. Both changing stoichiometry x and applying hydrostatic pressure P can tune the transition [6]; the pertinent P-T phase diagram for x 0.44 is shown in Fig.…”

“…1 [7]. Both Se substitution and applied pressure increase the bandwidth without moving the system away from half filling.Pressure tuning of the transition yields a critical form s͑0͒ ϳ ͓͑P 2 P c ͒͞P c ͔ m with the conductivity exponent m 1.1 6 0.2, while temperature scans for T , 0.8 K very close to P c find a critical dynamical response ͓s 2 s͑0͔͒ ϳ T 0.2260.02 [5]. Within the finite-size scaling picture of quantum phase transitions [8], these results give zn 4.6 6 0.4, where n characterizes the divergence of the spatial correlation length and the dynamical critical exponent z relates the temporal and spatial dimensions.…”