46 research outputs found
Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains
By means of thermal expansion and specific heat measurements on the
high-pressure phase of (VO)PO, the effects of two energy scales of
the weakly dimerized antiferromagnetic = 1/2 Heisenberg chain are explored.
The low energy scale, given by the spin gap , is found to manifest
itself in a pronounced thermal expansion anomaly. A quantitative analysis,
employing T-DMRG calculations, shows that this feature originates from changes
in the magnetic entropy with respect to , . This term, inaccessible by specific heat, is visible only in the
weak-dimerization limit where it reflects peculiarities of the excitation
spectrum and its sensitivity to variations in .Comment: 4 pages, 4 figures now identical with finally published versio
Thermally activated Peierls dimerization in ferromagnetic spin chains
We demonstrate that a Peierls dimerization can occur in ferromagnetic spin
chains activated by thermal fluctuations. The dimer order parameter and
entanglement measures are studied as functions of the modulation of the
magnetic exchange interaction and temperature, using a spin-wave theory and the
density-matrix renormalization group. We discuss the case where a periodic
modulation is caused by spin-phonon coupling and the case where electronic
states effectively induce such a modulation. The importance of the latter for a
number of transition metal oxides is highlighted.Comment: 4 pages, 4 figure
Dynamical correlation functions of the XXZ model at finite temperature
Combining a lattice path integral formulation for thermodynamics with the
solution of the quantum inverse scattering problem for local spin operators, we
derive a multiple integral representation for the time-dependent longitudinal
correlation function of the spin-1/2 Heisenberg XXZ chain at finite temperature
and in an external magnetic field. Our formula reproduces the previous results
in the following three limits: the static, the zero-temperature and the XY
limits.Comment: 22 pages, v4: typos corrected, published versio
Caracterização morfológica de calos de Eucalyptus urophylla S. T. Blake sob concentraçÔes de boro e cålcio
Dual ancestries and ecologies of the Late Glacial Palaeolithic in Britain
Genetic investigations of Upper Palaeolithic Europe have revealed a complex and transformative history of human population movements and ancestries, with evidence of several instances of genetic change across the European continent in the period following the Last Glacial Maximum (LGM). Concurrent with these genetic shifts, the post-LGM period is characterized by a series of significant climatic changes, population expansions and cultural diversification. Britain lies at the extreme northwest corner of post-LGM expansion and its earliest Late Glacial human occupation remains unclear. Here we present genetic data from Palaeolithic human individuals in the United Kingdom and the oldest human DNA thus far obtained from Britain or Ireland. We determine that a Late Upper Palaeolithic individual from Gough's Cave probably traced all its ancestry to Magdalenian-associated individuals closely related to those from sites such as El MirĂłn Cave, Spain, and TroisiĂšme Caverne in Goyet, Belgium. However, an individual from Kendrick's Cave shows no evidence of having ancestry related to the Goughâs Cave individual. Instead, the Kendrickâs Cave individual traces its ancestry to groups who expanded across Europe during the Late Glacial and are represented at sites such as Villabruna, Italy. Furthermore, the individuals differ not only in their genetic ancestry profiles but also in their mortuary practices and their diets and ecologies, as evidenced through stable isotope analyses. This finding mirrors patterns of dual genetic ancestry and admixture previously detected in Iberia but may suggest a more drastic genetic turnover in northwestern Europe than in the southwest
Initial Upper Palaeolithic Homo sapiens from Bacho Kiro Cave, Bulgaria
The Middle to Upper Palaeolithic transition in Europe witnessed the replacement and partial absorption of local Neanderthal populations by Homo sapiens populations of African origin. However, this process probably varied across regions and its details remain largely unknown. In particular, the duration of chronological overlap between the two groups is much debated, as are the implications of this overlap for the nature of the biological and cultural interactions between Neanderthals and H. sapiens. Here we report the discovery and direct dating of human remains found in association with Initial Upper Palaeolithic artefacts, from excavations at Bacho Kiro Cave (Bulgaria). Morphological analysis of a tooth and mitochondrial DNA from several hominin bone fragments, identified through proteomic screening, assign these finds to H. sapiens and link the expansion of Initial Upper Palaeolithic technologies with the spread of H. sapiens into the mid-latitudes of Eurasia before 45 thousand years ago. The excavations yielded a wealth of bone artefacts, including pendants manufactured from cave bear teeth that are reminiscent of those later produced by the last Neanderthals of western Europe. These finds are consistent with models based on the arrival of multiple waves of H. sapiens into Europe coming into contact with declining Neanderthal populations