217 research outputs found
Similarity of Fermi Surface in the Hidden Order State and in the Antiferromagnetic State of URu2Si2
Shubnikov-de Haas measurements of high quality URu2Si2 single crystals reveal
two previously unobserved Fermi surface branches in the so-called hidden order
phase. Therefore about 55% of the enhanced mass is now detected. Under pressure
in the antiferromagnetic state, the Shubnikov-de Haas frequencies for magnetic
fields applied along the crystalline c axis show little change compared with
the zero pressure data. This implies a similar Fermi surface in both the hidden
order and antiferromagnetic states, which strongly suggests that the lattice
doubling in the antiferromagnetic phase due to the ordering vector QAF = (0 0
1) already occurs in the hidden order. These measurements provide a good test
for existing or future theories of the hidden order parameter.Comment: 4 pages, 4 figure
Suppression of hidden order in URu2Si2 under pressure and restoration in magnetic field
We describe here recent inelastic neutron scattering experiments on the heavy
fermion compound URu2Si2 realized in order to clarify the nature of the hidden
order (HO) phase which occurs below T_0 = 17.5 K at ambient pressure. The
choice was to measure at a given pressure P where the system will go, by
lowering the temperature, successively from paramagnetic (PM) to HO and then to
antiferromagnetic phase (AF). Furthermore, in order to verify the selection of
the pressure, a macroscopic detection of the phase transitions was also
achieved in situ via its thermal expansion response detected by a strain gauge
glued on the crystal. Just above P_x = 0.5 GPa, where the ground state switches
from HO to AF, the Q_0 = (1, 0, 0) excitation disappears while the excitation
at the incommensurate wavevector Q_1 = (1.4, 0, 0) remains. Thus, the Q_0 = (1,
0, 0) excitation is intrinsic only in the HO phase. This result is reinforced
by studies where now pressure and magnetic field can be used as tuning
variable. Above P_x, the AF phase at low temperature is destroyed by a magnetic
field larger than H_AF (collapse of the AF Q_0 = (1, 0, 0) Bragg reflection).
The field reentrance of the HO phase is demonstrated by the reappearance of its
characteristic Q_0 = (1, 0, 0) excitation. The recovery of a PM phase will only
be achieved far above H_AF at H_M approx 35 T. To determine the P-H-T phase
diagram of URu2Si2, macroscopic measurements of the thermal expansion were
realized with a strain gauge. The reentrant magnetic field increases strongly
with pressure. Finally, to investigate the interplay between superconductivity
(SC) and spin dynamics, new inelastic neutron scattering experiments are
reported down to 0.4 K, far below the superconducting critical temperature T_SC
approx 1.3 K as measured on our crystal by diamagnetic shielding.Comment: 5 pages, 7 figures, ICN 2009 conference proceeding
Inelastic contribution of the resistivity in the hidden order in URu2Si2
In the hidden order of URu2Si2 the resistivity at very low temperature shows
no T^2 behavior above the transition to superconductivity. However, when
entering the antiferromagnetic phase, the Fermi liquid behavior is recovered.
We discuss the change of the inelastic term when entering the AF phase with
pressure considering the temperature dependence of the Grueneisen parameter at
ambient pressure and the influence of superconductivity by an extrapolation of
high field data.Comment: 5 pages, 2 figures, SCES conference proceedin
The IRAS 1-Jy Survey of Ultraluminous Infrared Galaxies: I. The sample and Luminosity Function
A complete flux-limited sample of 118 ultraluminous infrared galaxies (ULIGs)
has been identified from the IRAS Faint Source Catalog (FSC). The selection
criteria were a 60 micron flux density greater than 1 Jy in a region of the sky
delta > -40 deg, |b| > 30 deg. All sources were subsequently reprocessed using
coadded IRAS maps in order to obtain the best available flux estimates in all
four IRAS wavelength bands. The maximum observed infrared luminosity is L_ir =
10^{12.90} L_{sun}, and the maximum redshift is z = 0.268. The luminosity
function for ULIGs over the decade luminosity range L_ir = 10^{12} - 10^{13}
L_{sun} can be approximated by a power law Phi (L) ~= L^{-2.35} Mpc^{-3}
mag^{-1}. In the local Universe z < 0.1, the space density of ULIGs appears to
be comparable to or slightly larger than that of optically selected QSOs at
comparable bolometric luminosities. A maximum likelihood test suggests strong
evolution for our sample; assuming density evolution proportional to
(1+z)^{alpha} we find alpha = 7.6+/-3.2. Examination of the two-point
correlation function shows a barely significant level of clustering, xi (r) =
1.6 +/- 1.2, on size scales r ~= 22 h^{-1} Mpc.Comment: 18 pages of text, 10 pages of figures 1 to 6, 6 pages of tables 1 to
3, ApJS accepte
Angular Dependence of the High-Magnetic-Field Phase Diagram of URu2Si2
We present measurements of the magnetoresistivity RHOxx of URu2Si2 single
crystals in high magnetic fields up to 60 T and at temperatures from 1.4 K to
40 K. Different orientations of the magnetic field have been investigated
permitting to follow the dependence on Q of all magnetic phase transitions and
crossovers, where Q is the angle between the magnetic field and the easy-axis
c. We find out that all magnetic transitions and crossovers follow a simple
1/cos(Q) -law, indicating that they are controlled by the projection of the
field on the c-axis
Complex Landau Ginzburg Theory of the Hidden Order in URu_2Si_2
We develop a Landau Ginzburg theory of the hidden order phase and the local
moment antiferromagnetic phase of URu_2Si_2. We unify the two broken symmetries
in a common complex order parameter and derive many experimentally relevant
consequences such as the topology of the phase diagram in magnetic field and
pressure. The theory accounts for the appearance of a moment under application
of stress and the thermal expansion anomaly across the phase transitions. It
identifies the low energy mode which is seen in the hidden order phase near the
conmensurate wavector (0,0, 1) as the pseudo-Goldstone mode of the approximate
U(1) symmetry.Comment: 4 pages, 3 figure
Field Reentrance of the Hidden Order State of URu2Si2 under Pressure
Combination of neutron scattering and thermal expansion measurements under
pressure shows that the so-called hidden order phase of URu2Si2 reenters in
magnetic field when antiferromagnetism (AF) collapses at H_AF (T). Macroscopic
pressure studies of the HO-AF boundaries were realized at different pressures
via thermal expansion measurements under magnetic field using a strain gauge.
Microscopic proof at a given pressure is the reappearance of the resonance at
Q_0=(1,0,0) under field which is correlated with the collapse of the AF Bragg
reflections at Q_0.Comment: 5 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp
New Phase Induced by Pressure in the Iron-Arsenide Superconductor K-Ba122
The electrical resistivity rho of the iron-arsenide superconductor
Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four
underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic
ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases
linearly with pressure. At pressures above around 1.0 GPa, a second sharp
anomaly is detected at a lower temperature T_0, which rises with pressure. We
attribute this second anomaly to the onset of a phase that causes a
reconstruction of the Fermi surface. This new phase expands with increasing x
and it competes with superconductivity. We discuss the possibility that a
second spin-density wave orders at T_0, with a Q vector distinct from that of
the spin-density wave that sets in at T_N.Comment: Two higher K concentrations were added, revealing a steady expansion
of the new phase in the T-P phase diagra
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