74,381 research outputs found
Spin Gaps in a Frustrated Heisenberg model for CaVO
I report results of a density matrix renormalization group (DMRG) study of a
model for the two dimensional spin-gapped system CaVO. This study
represents the first time that DMRG has been used to study a two dimensional
system on large lattices, in this case as large as , allowing
extrapolation to the thermodynamic limit. I present a substantial improvement
to the DMRG algorithms which makes these calculations feasible.Comment: 10 pages, with 4 Postscript figure
On Deriving Space-Time From Quantum Observables and States
We prove that, under suitable assumptions, operationally motivated data
completely determine a space-time in which the quantum systems can be
interpreted as evolving. At the same time, the dynamics of the quantum system
is also determined. To minimize technical complications, this is done in the
example of three-dimensional Minkowski space.Comment: 19 pages, to appear in Communications in Mathematical Physics; minor
corrections mad
Control System Design Philosophy for Effective Operations and Maintenance
A well-designed control system facilitates the functions of machine
operation, maintenance and development. In addition, the overall effectiveness
of the control system can be greatly enhanced by providing reliable mechanisms
for coordination and communication, ensuring that these functions work in
concert. For good operability, the information presented to operators should be
consistent, easy to understand and customizable. A maintainable system is
segmented appropriately, allowing a broken element to be quickly identified and
repaired while leaving the balance of the system available. In a research and
development environment, the control system must meet the frequently changing
requirements of a variety of customers. This means the system must be flexible
enough to allow for ongoing modifications with minimal disruptions to
operations. Beyond the hardware and software elements of the control system,
appropriate workflow processes must be in place to maximize system uptime and
allow people to work efficiently. Processes that provide automatic electronic
communication ensure that information is not lost and reaches its destination
in a timely fashion. This paper discusses how these control system design and
quality issues have been applied at the Thomas Jefferson National Accelerator
Facility.Comment: ICALEPCS 200
Using A Nameserver to Enhance Control System Efficiency
The Thomas Jefferson National Accelerator Facility (Jefferson Lab) control
system uses a nameserver to reduce system response time and to minimize the
impact of client name resolution on front-end computers. The control system is
based on the Experimental Physics and Industrial Control System (EPICS), which
uses name-based broadcasts to initiate data communication. By default, when
EPICS process variables (PV) are requested by client applications, all
front-end computers receive the broadcasts and perform name resolution
processing against local channel name lists. The nameserver is used to offload
the name resolution task to a single node. This processing, formerly done on
all front-end computers, is now done only by the nameserver. In a control
system with heavily loaded front-end computers and high peak client connection
loads, a significant performance improvement is seen. This paper describes the
name server in more detail, and discusses the strengths and weaknesses of
making name resolution a centralized service.Comment: ICALEPCS 200
Energetics of Domain Walls in the 2D t-J model
Using the density matrix renormalization group, we calculate the energy of a
domain wall in the 2D t-J model as a function of the linear hole density
\rho_\ell, as well as the interaction energy between walls, for J/t=0.35. Based
on these results, we conclude that the ground state always has domain walls for
dopings 0 < x < 0.3. For x < 0.125, the system has (1,0) domain walls with
\rho_\ell ~ 0.5, while for 0.125 < x < 0.17, the system has a possibly
phase-separated mixture of walls with \rho_\ell ~ 0.5 and \rho_\ell =1. For x >
0.17, there are only walls with \rho_\ell =1. For \rho_\ell = 1, diagonal (1,1)
domain walls have very nearly the same energy as (1,0) domain walls.Comment: Several minor changes. Four pages, four encapsulated figure
Phase diagram of the half-filled Hubbard chain with next-nearest-neighbor hopping
We investigate the ground-state phase diagram of the half-filled
one-dimensional Hubbard model with next-nearest-neighbor hopping using the
Density-Matrix Renormalization Group technique as well as an unrestricted
Hartree-Fock approximation. We find commensurate and incommensurate disordered
magnetic insulating phases and a spin-gapped metallic phase in addition to the
one-dimensional Heisenberg phase. At large on-site Coulomb repulsion , we
make contact with the phase diagram of the frustrated Heisenberg chain, which
has spin-gapped phases for sufficiently large frustration. For weak ,
sufficiently large next-nearest-neighbor hopping leads to a band
structure with four Fermi points rather than two, producing a spin-gapped
metallic phase. As is increased in this regime, the system undergoes a
Mott-Hubbard transition to a frustrated antiferromagnetic insulator
A Renormalization Group Method for Quasi One-dimensional Quantum Hamiltonians
A density-matrix renormalization group (DMRG) method for highly anisotropic
two-dimensional systems is presented. The method consists in applying the usual
DMRG in two steps. In the first step, a pure one dimensional calculation along
the longitudinal direction is made in order to generate a low energy
Hamiltonian. In the second step, the anisotropic 2D lattice is obtained by
coupling in the transverse direction the 1D Hamiltonians. The method is applied
to the anisotropic quantum spin half Heisenberg model on a square lattice.Comment: 4 pages, 4 figure
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Informing the development of services supporting self-care for long term mental health conditions: A mixed method study of community based mental health initiatives in England
Background
Supporting self-care is being explored across health care systems internationally as an approach to improving care for long term conditions in the context of ageing populations and economic constraint. UK health policy advocates a range of approaches to supporting self-care, including the application of generic self-management type programmes across conditions. Within mental health, the scope of self-care remains poorly conceptualised and the existing evidence base for supporting self-care is correspondingly disparate. This paper aims to inform the development of support for self-care in mental health by considering how generic self-care policy guidance is implemented in the context of services supporting people with severe, long term mental health problems.
Methods
A mixed method study was undertaken comprising standardised psychosocial measures, questionnaires about health service use and qualitative interviews with 120 new referrals to three contrasting community based initiatives supporting self-care for severe, long term mental health problems, repeated nine months later. A framework approach was taken to qualitative analysis, an exploratory statistical analysis sought to identify possible associations between a range of independent variables and self-care outcomes, and a narrative synthesis brought these analyses together.
Results
Participants reported improvement in self-care outcomes (e.g. greater empowerment; less use of Accident and Emergency services). These changes were not associated with level of engagement with self-care support. Level of engagement was associated with positive collaboration with support staff. Qualitative data described the value of different models of supporting self-care and considered challenges. Synthesis of analyses suggested that timing support for self-care, giving service users control over when and how they accessed support, quality of service user-staff relationships and decision making around medication are important issues in supporting self-care in mental health.
Conclusions
Service delivery components – e.g. peer support groups, personal planning – advocated in generic self-care policy have value when implemented in a mental health context. Support for self-care in mental health should focus on core, mental health specific qualities; issues of control, enabling staff-service user relationships and shared decision making. The broad empirical basis of our research indicates the wider relevance of our findings across mental health settings
Comment on "Kagome Lattice Antiferromagnet Stripped to Its Basics"
Density matrix renormalization group (DMRG) calculations on large systems (up
to 3096 spins) indicate that the ground state of the Heisenberg model on a
3-chain Kagome strip is spontaneously dimerized. This system has degenerate
ground states and a gap to triplet and singlet excitations. These results are
in direct contradiction with recent results of Azaria et al (Phys. Rev. Lett.
81, 1694 (1998)) and suggest a need for a reexamination of the underlying field
theory.Comment: 1 page, submitted to PR
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