15 research outputs found
A Scanned Perturbation Technique For Imaging Electromagnetic Standing Wave Patterns of Microwave Cavities
We have developed a method to measure the electric field standing wave
distributions in a microwave resonator using a scanned perturbation technique.
Fast and reliable solutions to the Helmholtz equation (and to the Schrodinger
equation for two dimensional systems) with arbitrarily-shaped boundaries are
obtained. We use a pin perturbation to image primarily the microwave electric
field amplitude, and we demonstrate the ability to image broken time-reversal
symmetry standing wave patterns produced with a magnetized ferrite in the
cavity. The whole cavity, including areas very close to the walls, can be
imaged using this technique with high spatial resolution over a broad range of
frequencies.Comment: To be published in Review of Scientific Instruments,September, 199
Activation Energy of Metastable Amorphous Ge2Sb2Te5 from Room Temperature to Melt
Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level
show an exponential decline with temperature matching with the steady-state
thin-film resistivity measured at 858 K (melting temperature). This suggests
that the free carrier activation mechanisms form a continuum in a large
temperature scale (300 K - 858 K) and the metastable amorphous phase can be
treated as a super-cooled liquid. The effective activation energy calculated
using the resistivity versus temperature data follow a parabolic behavior, with
a room temperature value of 333 meV, peaking to ~377 meV at ~465 K and reaching
zero at ~930 K, using a reference activation energy of 111 meV (3kBT/2) at
melt. Amorphous GST is expected to behave as a p-type semiconductor at Tmelt ~
858 K and transitions from the semiconducting-liquid phase to the
metallic-liquid phase at ~ 930 K at equilibrium. The simultaneous Seebeck (S)
and resistivity versus temperature measurements of amorphous-fcc mixed-phase
GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with
degenerate semiconductors, and the dS/dT and room temperature activation energy
show a linear correlation. The single-crystal fcc is calculated to have dS/dT =
0.153 {\mu}V/K for an activation energy of zero and a Fermi level 0.16 eV below
the valance band edge.Comment: 5 pages, 5 figure
First Experimental Evidence for Chaos-Assisted Tunneling in a Microwave Annular Billiard
We report on first experimental signatures for chaos-assisted tunneling in a
two-dimensional annular billiard. Measurements of microwave spectra from a
superconducting cavity with high frequency resolution are combined with
electromagnetic field distributions experimentally determined from a normal
conducting twin cavity with high spatial resolution to resolve eigenmodes with
properly identified quantum numbers. Distributions of so-called quasi-doublet
splittings serve as basic observables for the tunneling between whispering
gallery type modes localized to congruent, but distinct tori which are coupled
weakly to irregular eigenstates associated with the chaotic region in phase
space.Comment: 5 pages RevTex, 5 low-resolution figures (high-resolution figures:
http://linac.ikp.physik.tu-darmstadt.de/heiko/chaospub.html, to be published
in Phys. Rev. Let
Influence of diffraction on the spectrum and wavefunctions of an open system
In this paper, we demonstrate the existence and significance of diffractive
orbits in an open microwave billiard, both experimentally and theoretically.
Orbits that diffract off of a sharp edge of the system are found to have a
strong influence on the transmission spectrum of the system, especially in the
regime where there are no stable classical orbits. On resonance, the
wavefunctions are influenced by both classical and diffractive orbits. Off
resonance, the wavefunctions are determined by the constructive interference of
multiple transient, nonperiodic orbits. Experimental, numerical, and
semiclassical results are presented.Comment: 27 pages, 29 figures, and 3 tables. Submitted to Physical Review E. A
copy with higher resolution figures is available at
http://monsoon.harvard.edu/~hersch/papers.htm
Single-Atom Gating of Quantum State Superpositions
The ultimate miniaturization of electronic devices will likely require local
and coherent control of single electronic wavefunctions. Wavefunctions exist
within both physical real space and an abstract state space with a simple
geometric interpretation: this state space--or Hilbert space--is spanned by
mutually orthogonal state vectors corresponding to the quantized degrees of
freedom of the real-space system. Measurement of superpositions is akin to
accessing the direction of a vector in Hilbert space, determining an angle of
rotation equivalent to quantum phase. Here we show that an individual atom
inside a designed quantum corral can control this angle, producing arbitrary
coherent superpositions of spatial quantum states. Using scanning tunnelling
microscopy and nanostructures assembled atom-by-atom we demonstrate how single
spins and quantum mirages can be harnessed to image the superposition of two
electronic states. We also present a straightforward method to determine the
atom path enacting phase rotations between any desired state vectors. A single
atom thus becomes a real space handle for an abstract Hilbert space, providing
a simple technique for coherent quantum state manipulation at the spatial limit
of condensed matter.Comment: Published online 6 April 2008 in Nature Physics; 17 page manuscript
(including 4 figures) + 3 page supplement (including 2 figures);
supplementary movies available at http://mota.stanford.ed
Measurement of Wave Chaotic Eigenfunctions in the Time-Reversal Symmetry-Breaking Crossover Regime
We present experimental results on eigenfunctions of a wave chaotic system in the continuous crossover regime between time-reversal symmetric and time-reversal symmetry-broken states. The statistical properties of the eigenfunctions of a two-dimensional microwave resonator are analyzed as a function of an experimentally determined time-reversal symmetry-breaking parameter. We test four theories of onepoint eigenfunction statistics and introduce a new theory relating the one-point and two-point statistical properties in the crossover regime. We also find a universal correlation between the one-point and twopoint statistical parameters for the crossover eigenfunctions. PACS numbers: 05.45. Mt, 03.65.Sq, 11.30.Er, 84.40.Az Many complex quantum systems whose underlying classical behavior is chaotic can be described by treating their Hamiltonian matrix elements as random numbers which fluctuate around zero with a Gaussian distribution. There are universal statistical properties of the eigenvalues and eigenfunctions of these random matrices which depend only on the symmetries of the Hamiltonian. For instance, random matrix theory has been shown to be consistent with the statistical properties of nuclei [1], molecules [2], and two-dimensional quantum dots When time-reversal symmetry is present, wave chaotic systems have statistical properties described by a Gaussian orthogonal ensemble (GOE) of random matrices Here we address the evolution of eigenfunctions of semiclassical wave chaotic systems from the TRS to the TRSB limits. A considerable theoretical literature has developed proposing detailed descriptions of eigenvector statistics in the crossover regime, although little experimental data are available to test these theories. These theories treat only the evolution of the one-point statistical property of eigenfunction distribution, PÍ‘jCj 2 Í’, which quantifies the degree of probability density, jCj 2 , fluctuations in the eigenfunctions The experimental arrangement used to create and measure the wave chaotic eigenfunctions has been described previously We have found that the nonreciprocal property of the ferrite, hence the degree of TRSB, is a function of frequency of the eigenmode in a relatively narrow range of frequency 2482 0031-9007Íž00Íž85(12)Íž2482(4)$15.0
Modeling and impacts of the latent heat of phase change and specific heat for phase change materials
Thickness dependence of the amorphous-cubic and cubic-hexagonal phase transition temperatures of GeSbTe thin films on silicon nitride
The crystallization temperature of GeSbTe thin films with thicknesses between 11 and 87 nm on silicon nitride was studied through resistance versus temperature measurements. The amorphous-cubic phase transition occurs at similar to 150 degrees C for all films thicknesses, whereas the cubic-hexagonal phase transition temperature increases with film thickness, from similar to 200 degrees C for the 20 nm film to similar to 250 degrees C for the 87 nm film. The cubic-hexagonal transition occurs gradually for the 11 nm film. Implications for phase-change memory devices are discussed. (C) 2011 Elsevier B.V. All rights reserved