11,986 research outputs found
Frustrated spin- Heisenberg magnet on a square-lattice bilayer: High-order study of the quantum critical behavior of the ---- model
The zero-temperature phase diagram of the spin-
---- model on an -stacked square-lattice
bilayer is studied using the coupled cluster method implemented to very high
orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor
Heisenberg exchange interactions, of strengths and , respectively, are included in each layer. The two layers are
coupled via a NN interlayer Heisenberg exchange interaction with a strength
. The magnetic order parameter (viz.,
the sublattice magnetization) is calculated directly in the thermodynamic
(infinite-lattice) limit for the two cases when both layers have
antiferromagnetic ordering of either the N\'{e}el or the striped kind, and with
the layers coupled so that NN spins between them are either parallel (when
) to one another. Calculations
are performed at th order in a well-defined sequence of approximations,
which exactly preserve both the Goldstone linked cluster theorem and the
Hellmann-Feynman theorem, with . The sole approximation made is to
extrapolate such sequences of th-order results for to the exact limit,
. By thus locating the points where vanishes, we calculate
the full phase boundaries of the two collinear AFM phases in the
-- half-plane with . In particular, we provide the
accurate estimate, (), for the
position of the quantum triple point (QTP) in the region . We also
show that there is no counterpart of such a QTP in the region ,
where the two quasiclassical phase boundaries show instead an ``avoided
crossing'' behavior, such that the entire region that contains the nonclassical
paramagnetic phases is singly connected
Ground-state phase diagram of the spin-1/2 square-lattice J1-J2 model with plaquette structure
Using the coupled cluster method for high orders of approximation and Lanczos
exact diagonalization we study the ground-state phase diagram of a quantum
spin-1/2 J1-J2 model on the square lattice with plaquette structure. We
consider antiferromagnetic (J1>0) as well as ferromagnetic (J1<0)
nearest-neighbor interactions together with frustrating antiferromagnetic
next-nearest-neighbor interaction J2>0. The strength of inter-plaquette
interaction lambda varies between lambda=1 (that corresponds to the uniform
J1-J2 model) and lambda=0 (that corresponds to isolated frustrated 4-spin
plaquettes). While on the classical level (s \to \infty) both versions of
models (i.e., with ferro- and antiferromagnetic J1) exhibit the same
ground-state behavior, the ground-state phase diagram differs basically for the
quantum case s=1/2. For the antiferromagnetic case (J1 > 0) Neel
antiferromagnetic long-range order at small J2/J1 and lambda \gtrsim 0.47 as
well as collinear striped antiferromagnetic long-range order at large J2/J1 and
lambda \gtrsim 0.30 appear which correspond to their classical counterparts.
Both semi-classical magnetic phases are separated by a nonmagnetic quantum
paramagnetic phase. The parameter region, where this nonmagnetic phase exists,
increases with decreasing of lambda. For the ferromagnetic case (J1 < 0) we
have the trivial ferromagnetic ground state at small J2/|J1|. By increasing of
J2 this classical phase gives way for a semi-classical plaquette phase, where
the plaquette block spins of length s=2 are antiferromagnetically long-range
ordered. Further increasing of J2 then yields collinear striped
antiferromagnetic long-range order for lambda \gtrsim 0.38, but a nonmagnetic
quantum paramagnetic phase lambda \lesssim 0.38.Comment: 10 pages, 15 figure
Performance of a First-Level Muon Trigger with High Momentum Resolution Based on the ATLAS MDT Chambers for HL-LHC
Highly selective first-level triggers are essential to exploit the full
physics potential of the ATLAS experiment at High-Luminosity LHC (HL-LHC). The
concept for a new muon trigger stage using the precision monitored drift tube
(MDT) chambers to significantly improve the selectivity of the first-level muon
trigger is presented. It is based on fast track reconstruction in all three
layers of the existing MDT chambers, made possible by an extension of the
first-level trigger latency to six microseconds and a new MDT read-out
electronics required for the higher overall trigger rates at the HL-LHC. Data
from -collisions at is used to study the
minimal muon transverse momentum resolution that can be obtained using the MDT
precision chambers, and to estimate the resolution and efficiency of the
MDT-based trigger. A resolution of better than is found in all sectors
under study. With this resolution, a first-level trigger with a threshold of
becomes fully efficient for muons with a transverse momentum
above in the barrel, and above in the
end-cap region.Comment: 6 pages, 11 figures; conference proceedings for IEEE NSS & MIC
conference, San Diego, 201
A solvable model of a random spin-1/2 XY chain
The paper presents exact calculations of thermodynamic quantities for the
spin-1/2 isotropic XY chain with random lorentzian intersite interaction and
transverse field that depends linearly on the surrounding intersite
interactions.Comment: 14 pages (Latex), 2 tables, 13 ps-figures included, (accepted for
publication in Phys.Rev.B
Development of Muon Drift-Tube Detectors for High-Luminosity Upgrades of the Large Hadron Collider
The muon detectors of the experiments at the Large Hadron Collider (LHC) have
to cope with unprecedentedly high neutron and gamma ray background rates. In
the forward regions of the muon spectrometer of the ATLAS detector, for
instance, counting rates of 1.7 kHz/square cm are reached at the LHC design
luminosity. For high-luminosity upgrades of the LHC, up to 10 times higher
background rates are expected which require replacement of the muon chambers in
the critical detector regions. Tests at the CERN Gamma Irradiation Facility
showed that drift-tube detectors with 15 mm diameter aluminum tubes operated
with Ar:CO2 (93:7) gas at 3 bar and a maximum drift time of about 200 ns
provide efficient and high-resolution muon tracking up to the highest expected
rates. For 15 mm tube diameter, space charge effects deteriorating the spatial
resolution at high rates are strongly suppressed. The sense wires have to be
positioned in the chamber with an accuracy of better than 50 ?micons in order
to achieve the desired spatial resolution of a chamber of 50 ?microns up to the
highest rates. We report about the design, construction and test of prototype
detectors which fulfill these requirements
Recent earthquakes near Whittier, California
This paper deals with recent earthquakes noted in a small area near
and to the south of Whittier, California, culminating in a shock at
8:46 a.m., P.S.T., on July 8, 1929, which just barely attained destructive
intensity. Since the establishment of the Seismological Laboratory
and the first group of auxiliary stations of the coordinated network in
southern California, installed in 1926 and 1927, earthquakes emanating
from this district have been registered as follows: a sharp shock at
11:14 a.m., P.S.T., on October 8, 1927; a swarm of small shocks on
December 30 and 31, 1928; and a group in May, 1929. Also, following
the main shock, many aftershocks occurred within a few hours, and
since then shocks have continued to occur, at increasingly long and
irregular intervals, down to the time of writing
Frustrated Heisenberg antiferromagnet on the honeycomb lattice: Spin gap and low-energy parameters
We use the coupled cluster method implemented to high orders of approximation
to investigate the frustrated spin- ----
antiferromagnet on the honeycomb lattice with isotropic Heisenberg interactions
of strength between nearest-neighbor pairs, between
next-nearest-neighbor pairs, and between next-next-neareast-neighbor
pairs of spins. In particular, we study both the ground-state (GS) and
lowest-lying triplet excited-state properties in the case , in the window of the frustration
parameter, which includes the (tricritical) point of maximum classical
frustration at . We present GS results for the
spin stiffness, , and the zero-field uniform magnetic susceptibility,
, which complement our earlier results for the GS energy per spin, ,
and staggered magnetization, , to yield a complete set of accurate
low-energy parameters for the model. Our results all point towards a phase
diagram containing two quasiclassical antiferromagnetic phases, one with N\'eel
order for , and the other with collinear striped order
for . The results for both and the spin gap
provide compelling evidence for a quantum paramagnetic phase that is
gapped over a considerable portion of the intermediate region , especially close to the two quantum critical points
at and . Each of our fully independent sets of
results for the low-energy parameters is consistent with the values
and , and with
the transition at being of continuous (and probably of the
deconfined) type and that at being of first-order type
Edge effects in graphene nanostructures: II. Semiclassical theory of spectral fluctuations and quantum transport
We investigate the effect of different edge types on the statistical
properties of both the energy spectrum of closed graphene billiards and the
conductance of open graphene cavities in the semiclassical limit. To this end,
we use the semiclassical Green's function for ballistic graphene flakes that we
have derived in Reference 1. First we study the spectral two point correlation
function, or more precisely its Fourier transform the spectral form factor,
starting from the graphene version of Gutzwiller's trace formula for the
oscillating part of the density of states. We calculate the two leading order
contributions to the spectral form factor, paying particular attention to the
influence of the edge characteristics of the system. Then we consider transport
properties of open graphene cavities. We derive generic analytical expressions
for the classical conductance, the weak localization correction, the size of
the universal conductance fluctuations and the shot noise power of a ballistic
graphene cavity. Again we focus on the effects of the edge structure. For both,
the conductance and the spectral form factor, we find that edge induced
pseudospin interference affects the results significantly. In particular
intervalley coupling mediated through scattering from armchair edges is the key
mechanism that governs the coherent quantum interference effects in ballistic
graphene cavities
Performance of the ATLAS Muon Drift-Tube Chambers at High Background Rates and in Magnetic Fields
The ATLAS muon spectrometer uses drift-tube chambers for precision tracking.
The performance of these chambers in the presence of magnetic field and high
radiation fluxes is studied in this article using test-beam data recorded in
the Gamma Irradiation Facility at CERN. The measurements are compared to
detailed predictions provided by the Garfield drift-chamber simulation
programme
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