T. F. Rosenbaum

Percolating through networks of random thresholds: Finite temperature electron tunneling in metal nanocrystal arrays

We investigate how temperature affects transport through large networks of nonlinear conductances with distributed thresholds. In monolayers of weakly coupled gold nanocrystals, quenched charge disorder produces a range of local thresholds for the onset of electron tunneling. Our measurements delineate two regimes separated by a crossover temperature T*. Up to T* the nonlinear zero-temperature shape of the current-voltage curves survives, but with a threshold voltage for conduction that decreases linearly with temperature. Above T* the threshold vanishes and the low-bias conductance increases rapidly with temperature. We develop a model that accounts for these findings and predicts T*.

Magnetoresistance in n- and p-type Ag2Te: Mechanisms and applications

We compare the large magnetoresistive response of slightly nonstoichiometric Ag_(2±δ)Te for a wide range of hole (p⩽8×10^(17)u200a cm^(−3)) and electron (n⩽4×10^(18)u200a cm^(−3))carrier densities. In the p-type material alone, a characteristic peak in the resistivity ρ(T,H) is dramatically enhanced and moves to higher temperature with increasing magnetic field, resulting in a high field (H∼5u200aT) magnetoresistance that is sizeable even at room temperature. By contrast, n-type specimens are geared for low-field (H<0.1u200aT) applications because of a striking linear field dependence of the magnetoresistance that appears to be restricted to the Ag-rich materials.

Nonsaturating magnetoresistance of inhomogeneous conductors: Comparison of experiment and simulation

The silver chalcogenides provide a striking example of the benefits of imperfection. Nanothreads of excess silver cause distortions in the current flow that yield a linear and nonsaturating transverse magnetoresistance (MR). Associated with the large and positive MR is a negative longitudinal MR. The longitudinal MR only occurs in the three-dimensional limit and thereby permits the determination of a characteristic length scale set by the spatial inhomogeneity. We find that this fundamental inhomogeneity length can be as large as 10μm. Systematic measurements of the diagonal and off-diagonal components of the resistivity tensor in various sample geometries show clear evidence of the distorted current paths posited in theoretical simulations. We use a random-resistor network model to fit the linear MR, and expand it from two to three dimensions to depict current distortions in the third (thickness) dimension. When compared directly to experiments on Ag_(2±δ)Se and Ag_(2±δ)Te, in magnetic fields up to 55T, the model identifies conductivity fluctuations due to macroscopic inhomogeneities as the underlying physical mechanism. It also accounts reasonably quantitatively for the various components of the resistivity tensor observed in the experiments.

Quantum and Classical Glass Transitions in LiHOxY1-xF4

When performed in the proper low-field, low-frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in a transverse field LiHo(x)Y(1-x)F(4) prove the existence of a spin-glass transition for x=0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.

Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott system V2O3

Magnetic correlations in all four phases of pure and doped vanadium sesquioxide (V_2O_3) have been examined by magnetic thermal-neutron scattering. Specifically, we have studied the antiferromagnetic and paramagnetic phases of metallic V_(2-y)O_3, the antiferromagnetic insulating and paramagnetic metallic phases of stoichiometric V_2O_3, and the antiferromagnetic and paramagnetic phases of insulating V_(1.944)Cr_(0.056)O_3. While the antiferromagnetic insulator can be accounted for by a localized Heisenberg spin model, the long-range order in the antiferromagnetic metal is an incommensurate spin-density wave, resulting from a Fermi surface nesting instability. Spin dynamics in the strongly correlated metal are dominated by spin fluctuations with a single lobe spectrum in the Stoner electron-hole continuum. Furthermore, our results in metallic V_2O_3 represent an unprecedentedly complete characterization of the spin fluctuations near a metallic quantum critical point, and provide quantitative support for the self-consistent renormalization theory for itinerant antiferromagnets in the small moment limit. Dynamic magnetic correlations for ħω<k_BT in the paramagnetic insulator carry substantial magnetic spectral weight. However, they are extremely short-ranged, extending only to the nearest neighbors. The phase transition to the antiferromagnetic insulator, from the paramagnetic metal and the paramagnetic insulator, introduces a sudden switching of magnetic correlations to a different spatial periodicity which indicates a sudden change in the underlying spin Hamiltonian. To describe this phase transition and also the unusual short-range order in the paramagnetic state, it seems necessary to take into account the orbital degrees of freedom associated with the degenerate d orbitals at the Fermi level in V_2O_3.

Local magnetometry at high fields and low temperatures using InAs Hall sensors

We characterize the temperature (0.3⩽T⩽300 K), magnetic field(0⩽H⩽80 kOe), and thickness (0.1, 0.5, and 2.5 μm) dependence of the Hall response of high purity InAs epilayers grown using metalorganic chemical vapor deposition. The high sensitivity, linearity, and temperature independence of the response make them attractive for local Hall probe magnetometry, and uniquely qualified for high field applications below liquid helium temperatures. As a stringent test of performance, we use a six element micron-sized array to monitor the internal field gradient during vortex avalanches at milliKelvin temperatures in a single crystal of YBa_2Cu_3O_(7−δ).

Barkhausen noise in the random field Ising magnet Nd_2Fe_(14)B

With sintered needles aligned and a magnetic field applied transverse to its easy axis, the rare-earth ferromagnet Nd

New phase boundary in highly correlated, barely metallic V_2O_3

_2

Vortex avalanches at one thousandth the superconducting transition temperature.

Fe

Vanishing magnetization relaxation in the high field quantum limit in YBa_2Cu_3O_(7-δ)

_{14}