5,190 research outputs found
SU(2)-invariant spin-1/2 Hamiltonians with RVB and other valence bond phases
We construct a family of rotationally invariant, local, S=1/2 Klein
Hamiltonians on various lattices that exhibit ground state manifolds spanned by
nearest-neighbor valence bond states. We show that with selected perturbations
such models can be driven into phases modeled by well understood quantum dimer
models on the corresponding lattices. Specifically, we show that the
perturbation procedure is arbitrarily well controlled by a new parameter which
is the extent of decoration of the reference lattice. This strategy leads to
Hamiltonians that exhibit i) RVB phases in two dimensions, ii) U(1) RVB
phases with a gapless ``photon'' in three dimensions, and iii) a Cantor
deconfined region in two dimensions. We also construct two models on the
pyrochlore lattice, one model exhibiting a RVB phase and the other a U(1)
RVB phase.Comment: 16 pages, 15 figures; 1 figure and some references added; some minor
typos fixe
Biot-Savart correlations in layered superconductors
We discuss the superconductor to normal phase transition in an
infinite-layered type-II superconductor in the limit where the Josephson
coupling between layers is negligible. We model each layer as a neutral gas of
thermally excited pancake vortices. We assume the dominant interaction between
vortices in the same and in different layers is the electromagnetic interaction
between the screening currents induced by these vortices. Our main result,
obtained by exactly solving the leading order renormalization group flow, is
that the phase transition in this model is a Kosterlitz--Thouless transition
despite being a three--dimensional system. While the transition itself is
driven by the unbinding of two-dimensional pancake vortices, an RG analysis of
the low temperature phase and a mean-field theory of the high temperature phase
reveal that both phases possess three-dimensional correlations. An experimental
consequence of this is that the jump in the measured in-plane superfluid
stiffness, which is a universal quantity in 2d Kosterlitz-Thouless theory, will
receive a small non--universal correction (of order 1% in
BiSrCaCuO). This overall picture places some claims
expressed in the literature on a more secure analytical footing and also
resolves some conflicting views.Comment: 16 pages, 2 figures; minor typos corrected, references adde
Unusual Lymphomas Developing in Chronic Lymphocytic Leukemia
We report three patients with chronic lymphocytic leukemia (CLL) who developed malignant lymphomas of unusual character and modes of presentation. Two of the patients had received low doses of chlorambucil for several years before they developed malignant lymphoma, diffuse, large cell type (LCL). In one of these patients LCL manifested as a grossly evident osteolytic lesion. In the second patient LCL developed initially as a localized lesion in the iliac bone. Both patients died within a few weeks after LCL was diagnosed. The third patient, who was found to have CLL during a routine examination, did not receive any therapy for the leukemia. Within six months the patient developed diffuse malignant lymphoma, mixed small and large cell type, with total extinction of the leukemic component. The disease responded favorably to chemotherapy for lymphoma, and the patient is alive with minimal residual disease at this time. Immunohistochemical studies in all three patients suggested transformation or dedifferentiation of the original neoplastic lymphoid clone rather than de novo appearance of another neoplasm
Selection from read-only memory with limited workspace
Given an unordered array of elements drawn from a totally ordered set and
an integer in the range from to , in the classic selection problem
the task is to find the -th smallest element in the array. We study the
complexity of this problem in the space-restricted random-access model: The
input array is stored on read-only memory, and the algorithm has access to a
limited amount of workspace. We prove that the linear-time prune-and-search
algorithm---presented in most textbooks on algorithms---can be modified to use
bits instead of words of extra space. Prior to our
work, the best known algorithm by Frederickson could perform the task with
bits of extra space in time. Our result separates
the space-restricted random-access model and the multi-pass streaming model,
since we can surpass the lower bound known for the latter
model. We also generalize our algorithm for the case when the size of the
workspace is bits, where . The running time
of our generalized algorithm is ,
slightly improving over the
bound of Frederickson's algorithm. To obtain the improvements mentioned above,
we developed a new data structure, called the wavelet stack, that we use for
repeated pruning. We expect the wavelet stack to be a useful tool in other
applications as well.Comment: 16 pages, 1 figure, Preliminary version appeared in COCOON-201
Observation of Superfluid Flow in a Bose-Einstein Condensed Gas
We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred
by a macroscopic object, a blue detuned laser beam, using nondestructive {\em
in situ} phase contrast imaging. A critical velocity for the onset of a
pressure gradient has been observed, and shown to be density dependent. The
technique has been compared to a calorimetric method used previously to measure
the heating induced by the motion of the laser beam.Comment: 4 pages, 5 figure
Closed Loop Testing of Microphonics Algorithms Using a Cavity Emulator
An analog crystal filter based cavity emulator is modified with reverse
biased varactor diodes to provide a tuning range of around 160 Hz. The piezo
drive voltage of the resonance controller is used to detune the cavity through
the bias voltage. A signal conditioning and summing circuit allows the
introduction of microphonics disturbance from a signal source or using real
microphonics data from cavity testing. This setup is used in closed loop with a
cavity controller and resonance controller to study the effectiveness of
resonance control algorithms suitable for superconducting cavities.Comment: Poster presented at LLRF Workshop 2023 (LLRF2023, arXiv: 2311.00901
LLRF System for the Fermilab PIP-II Superconducting LINAC
PIP-II is an 800 MEV superconducting linac that is in the initial
acceleration chain for the Fermilab accelerator complex. The RF system consists
of a warm front-end with an ion source, RFQ and buncher cavities along with 25
superconducting cryo-modules comprised of five different acceleration
. The LLRF system for the LINAC has to provide field and resonance
control for a total of 125 RF cavities.The LLRF system design is in the final
design review phase and will enter the production phase next year. The PIP-II
project is an international collaboration with various partner labs
contributing subsystems. The LLRF system design for the PIP-II Linac is
presented and the specification requirements and system performance in various
stages of testing are described in this paper.Comment: Talk presented at LLRF Workshop 2023 (LLRF2023, arXiv: 2311.00900
- …