157 research outputs found
Fermi Surface Reconstruction in CeRhCoIn
The evolution of the Fermi surface of CeRhCoIn was studied as
a function of Co concentration via measurements of the de Haas-van Alphen
effect. By measuring the angular dependence of quantum oscillation frequencies,
we identify a Fermi surface sheet with -electron character which undergoes
an abrupt change in topology as is varied. Surprisingly, this
reconstruction does not occur at the quantum critical concentration ,
where antiferromagnetism is suppressed to T=0. Instead we establish that this
sudden change occurs well below , at the concentration x ~ 0.4 where long
range magnetic order alters its character and superconductivity appears. Across
all concentrations, the cyclotron effective mass of this sheet does not
diverge, suggesting that critical behavior is not exhibited equally on all
parts of the Fermi surface.Comment: 4 pages, 4 figure
Nonvanishing Energy Scales at the Quantum Critical Point of CeCoIn5
Heat and charge transport were used to probe the magnetic field-tuned quantum
critical point in the heavy-fermion metal CeCoIn. A comparison of
electrical and thermal resistivities reveals three characteristic energy
scales. A Fermi-liquid regime is observed below , with both transport
coefficients diverging in parallel and as , the
critical field. The characteristic temperature of antiferromagnetic spin
fluctuations, , is tuned to a minimum but {\it finite} value at ,
which coincides with the end of the -linear regime in the electrical
resistivity. A third temperature scale, , signals the formation of
quasiparticles, as fermions of charge obeying the Wiedemann-Franz law.
Unlike , it remains finite at , so that the integrity of
quasiparticles is preserved, even though the standard signature of Fermi-liquid
theory fails.Comment: 4 pages, 4 figures (published version
The Faint End Slopes Of Galaxy Luminosity Functions In The COSMOS 2-Square Degree Field
We examine the faint-end slope of the rest-frame V-band luminosity function
(LF), with respect to galaxy spectral type, of field galaxies with redshift
z<0.5, using a sample of 80,820 galaxies with photometric redshifts in the
Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined,
the LF slope, alpha, ranges from -1.24 to -1.12, from the lowest redshift bin
to the highest. In the lowest redshift bin (0.02<z<0.1), where the magnitude
limit is M(V) ~ -13, the slope ranges from ~ -1.1 for galaxies with early-type
spectral energy distributions (SEDs), to ~ -1.9 for galaxies with
low-extinction starburst SEDs. In each galaxy SED category (Ell, Sbc, Scd/Irr,
and starburst), the faint-end slopes grow shallower with increasing redshift;
in the highest redshift bin (0.4<z<0.5), the slope is ~ -0.5 and ~ -1.3 for
early-types and starbursts respectively. The steepness of alpha at lower
redshift could be qualitatively explained by large numbers of faint dwarf
galaxies, perhaps of low surface brightness, which are not detected at higher
redshifts.Comment: 24 pages including 5 figures, accepted to ApJ
Superfluid Density and Field-Induced Magnetism in Ba(Fe1-xCox)2As2 and Sr(Fe1-xCox)2As2 Measured with Muon Spin Relaxation
We report muon spin rotation (SR) measurements of single crystal
Ba(FeCo)As and Sr(FeCo)As. From
measurements of the magnetic field penetration depth we find that for
optimally- and over-doped samples, varies monotonically
with the superconducting transition temperature T. Within the
superconducting state we observe a positive shift in the muon precession
signal, likely indicating that the applied field induces an internal magnetic
field. The size of the induced field decreases with increasing doping but is
present for all Co concentrations studied.Comment: 7 pages, accepted for publication in Phys. Rev.
Heavy nuclei at the end of the cosmic ray spectrum?
We provide an account of the possible acceleration of iron nuclei up to
energies EeV in the nearby, metally-rich starburst galaxy NGC 253. It
is suggested that particles can escape from the nuclear region with energies of
eV and then could be reaccelerated at the terminal shock of the
galactic superwind generated by the starburst, avoiding in this way the
photodisintegration expected if the nuclei were accelerated in the central
region of high photon density. We have also made estimates of the expected
arrival spectrum, which displays a strong dependency with the energy cutoff at
the source.Comment: Revised version, to appear in Physical Review
Insulating behavior in ultra-thin bismuth selenide field effect transistors
Ultrathin (~3 quintuple layer) field-effect transistors (FETs) of topological
insulator Bi2Se3 are prepared by mechanical exfoliation on 300nm SiO2/Si
susbtrates. Temperature- and gate-voltage dependent conductance measurements
show that ultrathin Bi2Se3 FETs are n-type, and have a clear OFF state at
negative gate voltage, with activated temperature-dependent conductance and
energy barriers up to 250 meV
High pressure transport properties of the topological insulator Bi2Se3
We report x-ray diffraction, electrical resistivity, and magnetoresistance
measurements on Bi2Se3 under high pressure and low temperature conditions.
Pressure induces profound changes in both the room temperature value of the
electrical resistivity as well as the temperature dependence of the
resistivity. Initially, pressure drives Bi2Se3 towards increasingly insulating
behavior and then, at higher pressures, the sample appears to enter a fully
metallic state coincident with a change in the crystal structure. Within the
low pressure phase, Bi2Se3 exhibits an unusual field dependence of the
transverse magnetoresistance that is positive at low fields and becomes
negative at higher fields. Our results demonstrate that pressures below 8 GPa
provide a non-chemical means to controllably reduce the bulk conductivity of
Bi2Se3
Role of electron-electron interactions in the charge dynamics of rare-earth-doped CaFe2As2
We have investigated the charge dynamics and the nature of many-body interactions in La- and Pr-doped CaFe2As2. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered at energy scales about one order of magnitude larger than the Fermi energy scale in these semimetals. The phenomena noted above have been observed previously in other correlated metals in which the dominant interactions are electronic in origin. Further evidence of significant electron-electron interactions is obtained from the presence of quadratic temperature and frequency-dependent terms in the scattering rate at low temperatures and frequencies in the uncollapsed tetragonal structures of La-doped and Pr-doped CaFe2As2. For temperatures below the structure collapse transition in Pr-doped CaFe2As2 at similar to 70 K, the scattering rate decreases due to weakening of electronic correlations, and the Drude spectral weight decreases due to modification of the low-energy electronic structure
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