3,346 research outputs found
Magnetism, structure, and charge correlation at a pressure-induced Mott-Hubbard insulator-metal transition
We use synchrotron x-ray diffraction and electrical transport under pressure
to probe both the magnetism and the structure of single crystal NiS2 across its
Mott-Hubbard transition. In the insulator, the low-temperature
antiferromagnetic order results from superexchange among correlated electrons
and couples to a (1/2, 1/2, 1/2) superlattice distortion. Applying pressure
suppresses the insulating state, but enhances the magnetism as the
superexchange increases with decreasing lattice constant. By comparing our
results under pressure to previous studies of doped crystals we show that this
dependence of the magnetism on the lattice constant is consistent for both band
broadening and band filling. In the high pressure metallic phase the lattice
symmetry is reduced from cubic to monoclinic, pointing to the primary influence
of charge correlations at the transition. There exists a wide regime of phase
separation that may be a general characteristic of correlated quantum matter.Comment: 5 pages, 3 figure
Universality and Critical Behavior at the Mott transition
We report conductivity measurements of Cr-doped V2O3 using a variable
pressure technique. The critical behavior of the conductivity near the
Mott-insulator to metal critical endpoint is investigated in detail as a
function of pressure and temperature. The critical exponents are determined, as
well as the scaling function associated with the equation of state. The
universal properties of a liquid-gas transition are found. This is potentially
a generic description of the Mott critical endpoint in correlated electron
materials.Comment: 3 figure
Zener double exchange from local valence fluctuations in magnetite
Magnetite (FeO) is a mixed valent system where electronic
conductivity occurs on the B-site (octahedral) iron sublattice of the spinel
structure. Below K, a metal-insulator transition occurs which is
argued to arise from the charge ordering of 2+ and 3+ iron valences on the
B-sites (Verwey transition). Inelastic neutron scattering measurements show
that optical spin waves propagating on the B-site sublattice (80 meV) are
shifted upwards in energy above due to the occurrence of B-B
ferromagnetic double exchange in the mixed valent metallic phase. The double
exchange interaction affects only spin waves of symmetry, not all
modes, indicating that valence fluctuations are slow and the double exchange is
constrained by electron correlations above .Comment: 4 pages, 5 figure
Dynamic phenomena in superconducting oxides by ESR
Dynamic electron spin resonance (ESR) measurements compare the paramagnetic and antiferromagnetic (AF) properties of superconducting oxides in the range 4 K to room temperature, at 8 MHz and 9.36 GHz. Two are derivatives of YBa2Cu30 7: 1: Nd(Nd0.05Ba0.95 )2Cu30 7, Te0 =72 K and II: Y0.2Cao.8Sr2[Cu2(Tlo.5Pb0.5 )]07, Te0 =108 K and two are cases where AF ordering dominates the weak superconductivity: III: Nb01.1\u3e 1. 25 ~Teo~ 10 K and IV: La2Ni04.00, 70 K :::: Teo:::: 40 K. At temperatures 298:::: T:::: 64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at Ke =2.06, gb =2.13, and Ka =2.24 are identified with the presence of 5% Nd3+( 41912 ) in the Ba layer because the characteristic Cu2+ impurity hyperfine structure is absent and the ESR signal disappears several degrees below Te. Near Te the ESR absorption is reduced by two orders of magnitude. Proximity effects give rise to interference fringes with period r1 ( T) independent of the field B and the rate of sweep dBzldt. ESR is observed below Te because flux penetrates the superconductor. The temperature dependence of r1 leads to an activation energy for the flux motion E0 (1)/R ~ 16 K and Ea (111)/R ~3 K =Te /4. In the superconducting state a coherent flux expulsion response to a change in B. from 500 mT to zero is observed in times T, = 8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through Te in a magnetic field, varies from Tnoise=3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: (i) For 3.5 K \u3c T \u3c T m T* \u3c Teo superconducting behavior was confirmed by the energy loss near zero magnetic field and the kinetics of high-field noise due to flux expulsion. Near g =2.00 ESR absorption is observed for all materials. A broad absorption near 50 to 100 mT at 9.36 GHz has been attributed to AF resonance. (ii) T m T* ~ T ~ Te identifies the range where flux motion gives rise to interference fringes in the ESR absorption. (iii) ESR and AF resonance are observed immediately after warming above Tc
Chemical analysis by X-ray spectroscopy near phase transitions in the solid state
The methods discussed in this work show that the types of changes which may be observed, by precise XAS measurements of Absorbance A versus temperature, across a phase transition are: the changes in the relaxation time of the final states due to fluctuations near a phase transition; the detection of the anomalous Bragg condition coupled to phonon modes XAS enhancement that identifies the temperature interval where the phonon modes are active, the symmetry changes which introduce new allowed transitions to finite states below an element edge, near Tc indicate what symmetry changes occur, and the method of XTDAFST0 = XAFS(T) - XAFS(T0), allows the precise measurement of the progressive changes in the Debye-Waller factor versus T near a phase transition, and identify (when no other structural changes occur, except in the vibrational modes of a specific bond) the bond responsible for the transition. The methods have been applied to the superconducting transition in layer cuprates and the metal to insulator transition in NiS2-xSex
Effect of correlations and disorder on electron states in the Mott-Hubbard insulator V_2O_3
We compare vanadium-deficient (nonstoichiometric) and titanium-doped vanadium sesquioxide through measurements of the electrical resistivity at a series of hydrostatic pressures, the magnetic susceptibility, and the low-temperature specific heat: all as a function of T. The pressure dependence of the critical temperature for this discontinuous metal-antiferromagnetic-insulator transition as well as the temperature dependence of the magnetic susceptibility track in the two cases. However, the pressure dependence of the Hubbard gap, the slower than exponential form of the low-temperature resistivity, and the concentration of two-level systems are markedly different for V_(1.9967)O_3 and (V_(0.99)Ti_(0.01))_2O_3. We discuss our results in terms of the intra-atomic Coulomb repulsion, which is of comparable magnitude to the bare bandwidth of the vanadium 3d states. The band splitting in the antiferromagnetic insulating state is argued to cross over to a Slater-type splitting between the subbands narrowed by correlations with a sufficient degree of oxygen nonstoichiometry or Ti doping
Nonlinear electric field effects at a continuous Mott-Hubbard transition
We characterize the non-Ohmic portion of the conductivity at temperatures T < 1 K in the highly correlated transition metal chalcogenide Ni(S, Se)_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_(-0.4)^(+0.3) Within the framework of finite temperature scaling, we find that the spatial correlation length exponent ν and the conductivity exponent μ differ
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