Nonlinear Electric Field Effects at a Continuous Mott-Hubbard Transition

Abstract

We characterize the non-Ohmic portion of the conductivity at temperatures T , 1 K in the highly correlated transition metal chalcogenide Ni͑S, Se͒ 2 . Pressure tuning of the T 0 metal-insulator transition reveals the influence of the quantum critical point and permits a direct determination of the dynamical critical exponent z 2.7 10.3 20.4 . Within the framework of finite temperature scaling, we find that the spatial correlation length exponent n and the conductivity exponent m differ. PACS numbers: 71.30. + h, 71.27. + a, 72.80.Ga Zero temperature phase transitions are fundamentally different from their finite temperature counterparts. Quantum fluctuations on a time scale set by Planck's constant and the characteristic energy of the system inextricably link the static and dynamic response. In the lexicon of continuous phase transitions, the static critical exponent which describes a thermodynamic property of a classical system must be joined in the quantum limit by a new dynamical exponent.The metal-insulator transition at zero temperature represents a particularly difficult limit of the problem. The T 0 electrical conductivity, s͑0͒, changes from zero in the insulator to a finite value in the metal, but it does not qualify as a thermodynamic quantity. Nonetheless, scaling arguments appear to be persuasive in the single electron limit [1,2]. Strong electron-electron interactions cloud the theoretical picture [3]. Fundamental questions-the role played by external electric and magnetic fields, the values of the static and dynamical exponents, the number of universality classes-remain unresolved.The experimental situation for highly correlated materials is complicated by the usual but unfortunate coincidence between electronic transitions and structural changes that shroud the critical behavior. 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. In the case of noninteracting electrons, Wegner scaling [1] asserts that m ͑d 2 2͒n n for three dimensions d. With interactions present, it is not possible to deconvolute z and n without additional experimental probes of the system.In this Letter, we charact...

Prediction of high-pressure polymorphism in NiS2at megabar pressures

“…[1][2][3][4]) and its insulator to metal transition induced either by pressure or by substituting S by Se (e.g. [5][6][7][8]). As shown in figure 1, NiS 2 is a paramagnetic charge transfer insulator at ambient conditions [9].…”

Unconventional resistivity at the border of metallic antiferromagnetism inNiS2

“…The identification of p c with the boundary of metallic antiferromagnetism is also suggested by the correspondence of the temperature-pressure and temperaturecomposition phase diagram of NiS 2 found in earlier work. 10,12,29,30 This predicted that the critical pressure for the border of metallic antiferromagnetism should be around 60 kbar, which is of the same order of magnitude as p c defined above. The identification of p c with the antiferromagnetic boundary in the metallic state is tentative and needs confirmation by other measurements and in particular the detection of a signature of T N in the resistivity for pressures below p c .…”

“…23,24 It can be metallized via the application of pressure 25,26,27,28 or by Se substitution. 10,12,25,27,28,29,30 The temperature-pressure phase diagram of NiS 2 and the temperature-composition phase diagram of NiS 2−x Se x (Fig. 1) are expected to be similar.…”

Electrical field induced shift of the Mott Metal-Insulator transition in thin films

“…3 These kind of transitions may be related to a charge transfer mechanism. A nonlinear dependence of the conductivity on the electrical field is reported for the highly correlated transition metal chalcogenide N i(S, Se) 2 and a continuous MIT is observed in this case 5 .…”