28,069 research outputs found
Frequency Evolution of Neutron Peaks Below Tc: Commensurate and Incommensurate Structure in LaSrCuO and YBaCuO
We study the evolution of the neutron cross-section with variable frequency
and fixed below in two different cuprate families. Our
calculations, which predominantly probe the role of d-wave pairing, lead to
generic features, independent of Fermi surface shapes. Among our findings,
reasonably consistent with experiment, are (i) for near the gap
energy , both optimal {LaSrCuO} and slightly underdoped YBCO exhibit
(comparably) incommensurate peaks (ii) peak sharpening below is seen in
{LaSrCuO}, (iii) quite generically, a frequency evolution from incommensurate
to commensurate and then back to incommensurate structure is found with
increasing . Due to their narrow regime of stability,
commensurate peaks in {LaSrCuO} should be extremely difficult to observe.Comment: RevTex 5pages, 4figures; Manuscript rewritten, figures revised, and
direct comparisons with experiments adde
Commensurate and Incommensurate Structure of the Neutron Cross Section in LaSrCuO and YBaCuO
We study the evolution of the d-wave neutron cross-section with variable
frequency \omega and fixed T (below and above Tc) in two different cuprate
families. The evolution from incommensurate to commensurate to incommensurate
peaks is rather generic within an RPA-like scheme. This behavior seems to be in
reasonable accord with experiments, and may help distinguish between this and
the "stripe" scenario.Comment: 2 pages; submitted to Proceedings of M2S-HTSC-V
Multidimensional entropy landscape of quantum criticality
The Third Law of Thermodynamics states that the entropy of any system in
equilibrium has to vanish at absolute zero temperature. At nonzero
temperatures, on the other hand, matter is expected to accumulate entropy near
a quantum critical point (QCP), where it undergoes a continuous transition from
one ground state to another. Here, we determine, based on general thermodynamic
principles, the spatial-dimensional profile of the entropy S near a QCP and its
steepest descent in the corresponding multidimensional stress space. We
demonstrate this approach for the canonical quantum critical compound
CeCu6-xAux near its onset of antiferromagnetic order. We are able to link the
directional stress dependence of S to the previously determined geometry of
quantum critical fluctuations. Our demonstration of the multidimensional
entropy landscape provides the foundation to understand how quantum criticality
nucleates novel phases such as high-temperature superconductivity.Comment: 14 pages, 4 figure
Unusual persistence of superconductivity against high magnetic fields in the strongly-correlated iron-chalcogenide film FeTe:O
We report an unusual persistence of superconductivity against high magnetic
fields in the iron chalcogenide film FeTe:O below ~ 2.5 K. Instead of
saturating like a mean-field behavior with a single order parameter, the
measured low-temperature upper critical field increases progressively,
suggesting a large supply of superconducting states accessible via magnetic
field or low-energy thermal fluctuations. We demonstrate that superconducting
states of finite momenta can be realized within the conventional theory,
despite its questionable applicability. Our findings reveal a fundamental
characteristic of superconductivity and electronic structure in the
strongly-correlated iron-based superconductors.Comment: 10 pages, 3 figure
Quantum criticality in spin chains with non-ohmic dissipation
We investigate the critical behavior of a spin chain coupled to bosonic baths
characterized by a spectral density proportional to , with .
Varying changes the effective dimension of the
system, where is the dynamical critical exponent and the number of spatial
dimensions is set to one. We consider two extreme cases of clock models,
namely Ising-like and U(1)-symmetric ones, and find the critical exponents
using Monte Carlo methods. The dynamical critical exponent and the anomalous
scaling dimension are independent of the order parameter symmetry for
all values of . The dynamical critical exponent varies continuously from for to for , and the anomalous scaling dimension
evolves correspondingly from to . The latter
exponent values are readily understood from the effective dimensionality of the
system being for , while for the anomalous
dimension takes the well-known exact value for the 2D Ising and XY models,
since then . A noteworthy feature is, however, that
approaches unity and approaches 1/4 for values of , while naive
scaling would predict the dissipation to become irrelevant for . Instead,
we find that for for both Ising-like and U(1)
order parameter symmetry. These results lead us to conjecture that for all
site-dissipative chains, these two exponents are related by the scaling
relation . We also connect our results to
quantum criticality in nondissipative spin chains with long-range spatial
interactions.Comment: 8 pages, 6 figure
Pengaruh Keikutsertaan Siswa Dalam Bimbingan Belajar Dan Ekstrakurikuler Terhadap Prestasi Belajar Matematika
Prestasi belajar merupakan suatu kemampuan atau keberhasilan belajar individu terhadap materi yang dipelajari, terlihat dari adanya Perubahan baik yang bersifat kognitif, afektif, maupun psikomotor. Untuk dapat meningkatkan prestasi belajar matematika siswa, maka diperlukan USAha untuk mencapai tujuan tersebut.Tujuan penelitian ini adalah untuk mengetahui pengaruh bimbingan belajar dan kegiatan ekstrakurikuler terhadap prestasi belajar matematika siswa
Hole dynamics in an antiferromagnet across a deconfined quantum critical point
We study the effects of a small density of holes, delta, on a square lattice
antiferromagnet undergoing a continuous transition from a Neel state to a
valence bond solid at a deconfined quantum critical point. We argue that at
non-zero delta, it is likely that the critical point broadens into a non-Fermi
liquid `holon metal' phase with fractionalized excitations. The holon metal
phase is flanked on both sides by Fermi liquid states with Fermi surfaces
enclosing the usual Luttinger area. However the electronic quasiparticles carry
distinct quantum numbers in the two Fermi liquid phases, and consequently the
limit of the ratio A_F/delta, as delta tends to zero (where A_F is the area of
a hole pocket) has a factor of 2 discontinuity across the quantum critical
point of the insulator. We demonstrate that the electronic spectrum at this
transition is described by the `boundary' critical theory of an impurity
coupled to a 2+1 dimensional conformal field theory. We compute the finite
temperature quantum-critical electronic spectra and show that they resemble
"Fermi arc" spectra seen in recent photoemission experiments on the pseudogap
phase of the cuprates.Comment: 33 pages, 8 figures, Longer version of cond-mat/0611536, with
additional results for electron spectrum at non-zero temperatur
- …