194 research outputs found
Quantum melting of incommensurate domain walls in two dimensions
Quantum fluctuations of periodic domain-wall arrays in two-dimensional
incommensurate states at zero temperature are investigated using the elastic
theory in the vicinity of the commensurate-incommensurate transition point.
Both stripe and honeycomb structures of domain walls with short-range
interactions are considered. It is revealed that the stripes melt and become a
stripe liquid in a large-wall-spacing (low-density) region due to dislocations
created by quantum fluctuations. This quantum melting transition is of second
order and characterized by the three-dimensional XY universality class.
Zero-point energies of the stripe and honeycomb structures are calculated. As a
consequence of these results, phase diagrams of the domain-wall solid and
liquid phases in adsorbed atoms on graphite are discussed for various
domain-wall masses. Quantum melting of stripes in the presence of long-range
interactions that fall off as power laws is also studied. These results are
applied to incommensurate domain walls in two-dimensional adsorbed atoms on
substrates and in doped antiferromagnets, e.g. cuprates and nickelates.Comment: 11 pages, 5 figure
Strong damping of phononic heat current by magnetic excitations in SrCu_2(BO_3)_2
Measurements of the thermal conductivity as a function of temperature and
magnetic field in the 2D dimer spin system SrCu(BO) are presented.
In zero magnetic field the thermal conductivity along and perpendicular to the
magnetic planes shows a pronounced double-peak structure as a function of
temperature. The low-temperature maximum is drastically suppressed with
increasing magnetic field. Our quantitative analysis reveals that the heat
current is due to phonons and that the double-peak structure arises from
pronounced resonant scattering of phonons by magnetic excitations.Comment: a bit more than 4 pages, 2 figures included; minor changes to improve
the clarity of the presentatio
Observation of two-magnon bound states in the two-leg ladders of (Ca,La)14Cu24O41
Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2
two-leg ladders of Ca_14-x La_x Cu_24 O_41 (x=5 and 4) for polarization of the
electrical field parallel to the legs and the rungs, respectively. Two peaks at
about 2140 and 2800 1/cm reflect van-Hove singularities in the density of
states of the strongly dispersing 2-magnon singlet bound state, and a broad
peak at about 4000 1/cm is identified with the 2-magnon continuum. Two
different theoretical approaches (Jordan-Wigner fermions and perturbation
theory) describe the data very well for J_parallel = 1050 - 1100 1/cm and
J_parallel / J_perp = 1 - 1.1. A striking similarity of the high-energy
continuum absorption of the ladders and of the undoped high T_c cuprates is
observed.Comment: 4 pages, 3 figures, Revte
Femtosecond control of electric currents at the interfaces of metallic ferromagnetic heterostructures
The idea to utilize not only the charge but also the spin of electrons in the
operation of electronic devices has led to the development of spintronics,
causing a revolution in how information is stored and processed. A novel
advancement would be to develop ultrafast spintronics using femtosecond laser
pulses. Employing terahertz (10 Hz) emission spectroscopy, we
demonstrate optical generation of spin-polarized electric currents at the
interfaces of metallic ferromagnetic heterostructures at the femtosecond
timescale. The direction of the photocurrent is controlled by the helicity of
the circularly polarized light. These results open up new opportunities for
realizing spintronics in the unprecedented terahertz regime and provide new
insights in all-optical control of magnetism.Comment: 3 figures and 2 tables in the main tex
Orbital order in the low-dimensional quantum spin system TiOCl probed by ESR
We present electron spin resonance data of Ti (3) ions in single
crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g
tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is
predominantly occupied and owing to the occurrence of orbital order a linear
spin chain forms along the crystallographic b axis. This result corroborates
recent theoretical LDA+U calculations of the band structure. The temperature
dependence of the parameters of the resonance signal suggests a strong coupling
between spin and lattice degrees of freedom and gives evidence for a transition
to a nonmagnetic ground state at 67 K.Comment: revised version, accepted for publication in Phys. Rev. B, Rapid Com
Adsorption Isotherms of Hydrogen: The Role of Thermal Fluctuations
It is shown that experimentally obtained isotherms of adsorption on solid
substrates may be completely reconciled with Lifshitz theory when thermal
fluctuations are taken into account. This is achieved within the framework of a
solid-on-solid model which is solved numerically. Analysis of the fluctuation
contributions observed for hydrogen adsorption onto gold substrates allows to
determine the surface tension of the free hydrogen film as a function of film
thickness. It is found to decrease sharply for film thicknesses below seven
atomic layers.Comment: RevTeX manuscript (3 pages output), 3 figure
Fractal Conductance Fluctuations in Gold--Nanowires
A detailed analysis of magneto-conductance fluctuations of quasiballistic
gold-nanowires of various lengths is presented. We find that the variance
when analyzed for much
smaller than the correlation field varies according to with indicating that the graph of
vs. is fractal. We attribute this behavior to the existence of
long-lived states arising from chaotic trajectories trapped close to regular
classical orbits. We find that decreases with increasing length of the
wires.Comment: 5 pages, Revtex with epsf, 4 Postscript figures, final version
accepted as Phys. Rev. Let
Strong anisotropy of superexchange in the copper-oxygen chains of La_{14-x}Ca_{x}Cu_{24}O_{41}
Electron spin resonance data of Cu^{2+} ions in La_{14-x}Ca_{x}Cu_{24}O_{41}
crystals (x=9,11,12) reveal a very large width of the resonance line in the
paramagnetic state. This signals an unusually strong anisotropy of ~10% of the
isotropic Heisenberg superexchange in the Cu-O chains of this compound. The
strong anisotropy can be explained by the specific geometry of two symmetrical
90 degree Cu-O-Cu bonds, which boosts the importance of orbital degrees of
freedom. Our data show the apparent limitations of the applicability of an
isotropic Heisenberg model to the low dimensional cuprates.Comment: 14 pages, 3 figures included, to be published in Phys. Rev. Let
'Reaching the hard to reach' - lessons learned from the VCS (voluntary and community Sector). A qualitative study.
Background The notion 'hard to reach' is a contested and ambiguous term that is commonly used within the spheres of social care and health, especially in discourse around health and social inequalities. There is a need to address health inequalities and to engage in services the marginalized and socially excluded sectors of society. Methods This paper describes a pilot study involving interviews with representatives from eight Voluntary and Community Sector (VCS) organisations . The purpose of the study was to explore the notion of 'hard to reach' and perceptions of the barriers and facilitators to accessing services for 'hard to reach' groups from a voluntary and community sector perspective. Results The 'hard to reach' may include drug users, people living with HIV, people from sexual minority communities, asylum seekers, refugees, people from black and ethnic minority communities, and homeless people although defining the notion of the 'hard to reach' is not straight forward. It may be that certain groups resist engaging in treatment services and are deemed hard to reach by a particular service or from a societal stance. There are a number of potential barriers for people who may try and access services, including people having bad experiences in the past; location and opening times of services and how services are funded and managed. A number of areas of commonality are found in terms of how access to services for 'hard to reach' individuals and groups could be improved including: respectful treatment of service users, establishing trust with service users, offering service flexibility, partnership working with other organisations and harnessing service user involvement.
Conclusions: If health services are to engage with groups that are deemed 'hard to reach' and marginalised from mainstream health services, the experiences and practices for engagement from within the VCS may serve as useful lessons for service improvement for statutory health services
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