511 research outputs found
Quantifying the azimuthal plasmaspheric density structure and dynamics inferred from IMAGE EUV
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95221/1/jgra22185.pd
Thermodynamics of free and bound magnons in graphene
Symmetry-broken electronic phases support neutral collective excitations. For
example, monolayer graphene in the quantum Hall regime hosts a nearly ideal
ferromagnetic phase at filling factor that spontaneously breaks spin
rotation symmetry. This ferromagnet has been shown to support spin-wave
excitations known as magnons which can be generated and detected electrically.
While long-distance magnon propagation has been demonstrated via transport
measurements, important thermodynamic properties of such magnon
populations--including the magnon chemical potential and density--have thus far
proven out of reach of experiments. Here, we present local measurements of the
electron compressibility under the influence of magnons, which reveal a
reduction of the gap by up to 20%. Combining these measurements with
estimates of the temperature, our analysis reveals that the injected magnons
bind to electrons and holes to form skyrmions, and it enables extraction of the
free magnon density, magnon chemical potential, and average skyrmion spin. Our
methods furnish a novel means of probing the thermodynamic properties of
charge-neutral excitations that is applicable to other symmetry-broken
electronic phases
Upgrade of a low-temperature scanning tunneling microscope for electron-spin resonance
Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy resolution of spin resonance spectroscopy with the atomic scale control and spatial resolution of STM. Here we describe the upgrade of a helium-3 STM with a 2D vector-field magnet (Bz = 8.0 T, Bx = 0.8 T) to an ESR-STM. The system is capable of delivering radio frequency (RF) power to the tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field-sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of (1.004 ± 0.001) μB. Our upgrade enables to toggle between a DC mode, where the STM is operated with the regular control electronics, and an ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe spectroscopy or reading of spin-states. Both modes allow for simultaneous radiofrequency excitation, which we add via a resistive pick-off tee to the bias voltage path. The RF cabling from room temperature to the 350 mK stage has an average attenuation of 18 dB between 5 and 25 GHz. The cable segment between the 350 mK stage and the STM tip presently attenuates an additional 34+5−3 dB from 10 to 26 GHz and 38+3−2 dB between 20 and 30 GHz. We discuss our transmission losses and indicate ways to reduce this attenuation. We finally demonstrate how to synchronize the arrival times of RF and DC pulses coming from different paths to the STM junction, a prerequisite for future pulsed ESR experiments
Unconventional sequence of correlated Chern insulators in magic-angle twisted bilayer graphene
The interplay between strong electron-electron interactions and band topology
can lead to novel electronic states that spontaneously break symmetries. The
discovery of flat bands in magic-angle twisted bilayer graphene (MATBG) with
nontrivial topology has provided a unique platform in which to search for new
symmetry-broken phases. Recent scanning tunneling microscopy and transport
experiments have revealed a sequence of topological insulating phases in MATBG
with Chern numbers near moir\'e band filling
factors , corresponding to a simple pattern of
flavor-symmetry-breaking Chern insulators. Here, we report high-resolution
local compressibility measurements of MATBG with a scanning single electron
transistor that reveal a new sequence of incompressible states with unexpected
Chern numbers observed down to zero magnetic field. We find that the Chern
numbers for eight of the observed incompressible states are incompatible with
the simple picture in which the bands are sequentially filled. We
show that the emergence of these unusual incompressible phases can be
understood as a consequence of broken translation symmetry that doubles the
moir\'e unit cell and splits each band into a band and a
band. Our findings significantly expand the known phase diagram of MATBG,
and shed light onto the origin of the close competition between different
correlated phases in the system
Asymptotics for products of characteristic polynomials in classical -Ensembles
We study the local properties of eigenvalues for the Hermite (Gaussian),
Laguerre (Chiral) and Jacobi -ensembles of random matrices.
More specifically, we calculate scaling limits of the expectation value of
products of characteristic polynomials as . In the bulk of the
spectrum of each -ensemble, the same scaling limit is found to be
whose exact expansion in terms of Jack polynomials is well
known. The scaling limit at the soft edge of the spectrum for the Hermite and
Laguerre -ensembles is shown to be a multivariate Airy function, which
is defined as a generalized Kontsevich integral. As corollaries, when
is even, scaling limits of the -point correlation functions for the three
ensembles are obtained. The asymptotics of the multivariate Airy function for
large and small arguments is also given. All the asymptotic results rely on a
generalization of Watson's lemma and the steepest descent method for integrals
of Selberg type.Comment: [v3] 35 pages; this is a revised and enlarged version of the article
with new references, simplified demonstations, and improved presentation. To
be published in Constructive Approximation 37 (2013
Measurement of and Production in Collisions at = 1.96 TeV
The Standard Model predictions for and production are
tested using an integrated luminosity of 200 pb of \ppbar collision data
collected at the Collider Detector at Fermilab. The cross sections are measured
selecting leptonic decays of the and bosons, and photons with
transverse energy GeV that are well separated from leptons. The
production cross sections and kinematic distributions for the and
are compared to SM predictions.Comment: 7 pages, 4 figures, submitted to PR
Measurement of the Lifetime Difference Between B_s Mass Eigenstates
We present measurements of the lifetimes and polarization amplitudes for B_s
--> J/psi phi and B_d --> J/psi K*0 decays. Lifetimes of the heavy (H) and
light (L) mass eigenstates in the B_s system are separately measured for the
first time by determining the relative contributions of amplitudes with
definite CP as a function of the decay time. Using 203 +/- 15 B_s decays, we
obtain tau_L = (1.05 +{0.16}/-{0.13} +/- 0.02) ps and tau_H = (2.07
+{0.58}/-{0.46} +/- 0.03) ps. Expressed in terms of the difference DeltaGamma_s
and average Gamma_s, of the decay rates of the two eigenstates, the results are
DeltaGamma_s/Gamma_s = (65 +{25}/-{33} +/- 1)%, and DeltaGamma_s = (0.47
+{0.19}/-{0.24} +/- 0.01) inverse ps.Comment: 8 pages, 3 figures, 2 tables; as published in Physical Review Letters
on 16 March 2005; revisions are for length and typesetting only, no changes
in results or conclusion
Search for the Higgs boson in events with missing transverse energy and b quark jets produced in proton-antiproton collisions at s**(1/2)=1.96 TeV
We search for the standard model Higgs boson produced in association with an
electroweak vector boson in events with no identified charged leptons, large
imbalance in transverse momentum, and two jets where at least one contains a
secondary vertex consistent with the decay of b hadrons. We use ~1 fb-1
integrated luminosity of proton-antiproton collisions at s**(1/2)=1.96 TeV
recorded by the CDF II experiment at the Tevatron. We find 268 (16) single
(double) b-tagged candidate events, where 248 +/- 43 (14.4 +/- 2.7) are
expected from standard model background processes. We place 95% confidence
level upper limits on the Higgs boson production cross section for several
Higgs boson masses ranging from 110 GeV/c2 to 140 GeV/c2. For a mass of 115
GeV/c2 the observed (expected) limit is 20.4 (14.2) times the standard model
prediction.Comment: 8 pages, 2 figures, submitted to Phys. Rev. Let
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