352 research outputs found
From stripe to checkerboard order on the square lattice in the presence of quenched disorder
We discuss the effects of quenched disorder on a model of charge density wave
(CDW) ordering on the square lattice. Our model may be applicable to the
cuprate superconductors, where a random electrostatic potential exists in the
CuO2 planes as a result of the presence of charged dopants. We argue that the
presence of a random potential can affect the unidirectionality of the CDW
order, characterized by an Ising order parameter. Coupling to a unidirectional
CDW, the random potential can lead to the formation of domains with 90 degree
relative orientation, thus tending to restore the rotational symmetry of the
underlying lattice. We find that the correlation length of the Ising order can
be significantly larger than the CDW correlation length. For a checkerboard CDW
on the other hand, disorder generates spatial anisotropies on short length
scales and thus some degree of unidirectionality. We quantify these disorder
effects and suggest new techniques for analyzing the local density of states
(LDOS) data measured in scanning tunneling microscopy experiments.Comment: 10 pages, 11 figures; added referenc
Evidence for gapped spin-wave excitations in the frustrated Gd2Sn2O7 pyrochlore antiferromagnet from low-temperature specific heat measurements
We have measured the low-temperature specific heat of the geometrically
frustrated pyrochlore Heisenberg antiferromagnet Gd2Sn2O7 in zero magnetic
field. The specific heat is found to drop exponentially below approximately 350
mK. This provides evidence for a gapped spin-wave spectrum due to an anisotropy
resulting from single ion effects and long-range dipolar interactions. The data
are well fitted by linear spin-wave theory, ruling out unconventional low
energy magnetic excitations in this system, and allowing a determination of the
pertinent exchange interactions in this material
Tonic-clonic seizures as a possible complication for cerebrospinal fluid leakage after intradural spinal surgery, a case report
Abstract Background Cerebrospinal fluid leakage is a well-known spinal surgery complication, especially in adults population. Pseudomeningocele is its most common manifestation and it can bring to some conditions, such as intracranial hypotension, infections and wound healing complication. Epilepsy is not classically associated to CSF leakage. We described a case of a female patient who developed tonic-clonic seizures associated with a pseudomeningocele after a detethering surgery. Case description A 16 year old female was admitted to our department for surgical treatment of a tethered cord for a sacral lipoma. Her medical history was remarkable mental retardation with psychiatric disturbs and hypothyroidism. She underwent a surgical intervention for the detethering of conus and dura was closed by a suture and fibrin glue. During third postoperative day she started to suffer a severe occipital headache, followed by tonic-clonic seizures. During suture removal, a collection suspected for a pseudomeningocele was found and chemical and radiological exams confirmed the dubious. Despite a continuous bed rest, collection continued to form. So, we decided to perform a surgical revision and to close dural defect. After intervention, patient did not suffer postural headache anymore and after 3 and 6 months she was found in good health. Conclusion We described the importance of significant morbidity, i.e. that of tonic clonic seizures as a sign of an occult CSF leakage after spinal surgery. Here, hydrocephalus as a condition was present, the change in pressure of CSF can determine seizures as well as promote this complication. A pseudomeningocele is not a trivial complication
Quantum spin fluctuations in the dipolar Heisenberg-like rare earth pyrochlores
The magnetic pyrochlore oxide materials of general chemical formula R2Ti2O7
and R2Sn2O7 (R = rare earth) display a host of interesting physical behaviours
depending on the flavour of rare earth ion. These properties depend on the
value of the total magnetic moment, the crystal field interactions at each rare
earth site and the complex interplay between magnetic exchange and long-range
dipole-dipole interactions. This work focuses on the low temperature physics of
the dipolar isotropic frustrated antiferromagnetic pyrochlore materials.
Candidate magnetic ground states are numerically determined at zero temperature
and the role of quantum spin fluctuations around these states are studied using
a Holstein-Primakoff spin wave expansion to order 1/S. The results indicate the
strong stability of the proposed classical ground states against quantum
fluctuations. The inclusion of long range dipole interactions causes a
restoration of symmetry and a suppression of the observed anisotropy gap
leading to an increase in quantum fluctuations in the ground state when
compared to a model with truncated dipole interactions. The system retains most
of its classical character and there is little deviation from the fully ordered
moment at zero temperature.Comment: Latex2e, 18 pages, 4 figures, IOP forma
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