822 research outputs found
Dynamical mean field theory of correlated gap formation in Pu monochalcogenides
The correlated Kondo insulator state of the plutonium monochalcogenides is
investigated using the dynamical mean field theory (DMFT) and the local density
approximation +U (LDA+U). The DMFT-dynamical fluctuations lead to a correlated
insulator state at elevated temperature, in sharp contrast to the static LDA+U
approach that fails to reproduce both the insulating behavior and anomalous
lattice constant. The DMFT conversely predicts the experimentally observed
anomalous increase of the gap with pressure and explains the lattice constant
very well.Comment: 4 pages, 4 figure
Switchable Hardening of a Ferromagnet at Fixed Temperature
The intended use of a magnetic material, from information storage to power
conversion, depends crucially on its domain structure, traditionally crafted
during materials synthesis. By contrast, we show that an external magnetic
field applied transverse to the preferred magnetization of a model disordered
uniaxial ferromagnet is an isothermal regulator of domain pinning. At elevated
temperatures, near the transition into the paramagnet, modest transverse fields
increase the pinning, stabilize the domain structure, and harden the magnet,
until a point where the field induces quantum tunneling of the domain walls and
softens the magnet. At low temperatures, tunneling completely dominates the
domain dynamics and provides an interpretation of the quantum phase transition
in highly disordered magnets as a localization/delocalization transition for
domain walls. While the energy scales of the rare earth ferromagnet studied
here restrict the effects to cryogenic temperatures, the principles discovered
are general and should be applicable to existing classes of highly anisotropic
ferromagnets with ordering at room temperature or above.Comment: 10 pages, 4 figure
Direct and alignment-insensitive measurement of cantilever curvature
We analytically derive and experimentally demonstrate a method for the
simultaneous measurement of deflection for large arrays of cantilevers. The
Fresnel diffraction patterns of a cantilever independently reveals tilt,
curvature, cubic and higher order bending of the cantilever. It provides a
calibrated absolute measurement of the polynomial coefficients describing the
cantilever shape, without careful alignment and could be applied to several
cantilevers simultaneously with no added complexity. We show that the method is
easily implemented, works in both liquid mediums and in air, for a broad range
of displacements and is especially suited to the requirements for multi-marker
biosensors.Comment: 5 Pages, 4 figures, letter forma
Probing many-body localization in a disordered quantum magnet
Quantum states cohere and interfere. Quantum systems composed of many atoms
arranged imperfectly rarely display these properties. Here we demonstrate an
exception in a disordered quantum magnet that divides itself into nearly
isolated subsystems. We probe these coherent clusters of spins by driving the
system beyond its linear response regime at a single frequency and measuring
the resulting "hole" in the overall linear spectral response. The Fano shape of
the hole encodes the incoherent lifetime as well as coherent mixing of the
localized excitations. For the disordered Ising magnet,
, the quality factor for spectral holes
can be as high as 100,000. We tune the dynamics of the quantum degrees of
freedom by sweeping the Fano mixing parameter through zero via the
amplitude of the ac pump as well as a static external transverse field. The
zero-crossing of is associated with a dissipationless response at the drive
frequency, implying that the off-diagonal matrix element for the two-level
system also undergoes a zero-crossing. The identification of localized
two-level systems in a dense and disordered dipolar-coupled spin system
represents a solid state implementation of many-body localization, pushing the
search forward for qubits emerging from strongly-interacting, disordered,
many-body systems.Comment: 22 pages, 6 figure
X-ray induced persistent photoconductivity in Si-doped AlGaAs
We demonstrate that X-ray irradiation can be used to induce an
insulator-metal transition in Si-doped AlGaAs, a
semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX}
centers into their shallow donor state was revealed by studying the
photoconductance along with fluorescence. The photoconductance as a function of
incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying
that core-hole excitation of Ga and As are efficient primary steps for the
excitation of {\it DX} centers. A high quantum yield () suggests that
the excitation is indirect and nonlocal, due to secondary electrons, holes, and
fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we
corrected i
Quantum and Classical Glass Transitions in
When performed in the proper low field, low frequency limits, measurements of
the dynamics and the nonlinear susceptibility in the model Ising magnet in
transverse field, , prove the existence
of a spin glass transition for = 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.Comment: 5 pages, 5 figures. Updated figure 3 with corrected calibration
information for thermometr
Scaling of magnetic fluctuations near a quantum phase transition
We use inelastic neutron scattering to measure the magnetic fluctuations in a
single crystal of the heavy fermion alloy CeCu_5.9Au_0.1 close to the
antiferromagnetic quantum critical point. The energy and temperature-dependent
spectra obey (E/T) scaling at Q near (1,0,0). The neutron data and earlier bulk
susceptibility are consistent with the form 1/X ~ f(Q)+(-iE+bT)^a, with an
anomalous exponent a=0.8. We confirm the earlier observation of quasi-low
dimensionality and show how both the magnetic fluctuations and the
thermodynamics can be understood in terms of a quantum Lifshitz point.Comment: Latex file with two postscript figure
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