14,612 research outputs found
Quantum critical behaviour of the plateau-insulator transition in the quantum Hall regime
High-field magnetotransport experiments provide an excellent tool to
investigate the plateau-insulator phase transition in the integral quantum Hall
effect. Here we review recent low-temperature high-field magnetotransport
studies carried out on several InGaAs/InP heterostructures and an InGaAs/GaAs
quantum well. We find that the longitudinal resistivity near the
critical filling factor ~ 0.5 follows the universal scaling law
, where . The critical exponent equals ,
which indicates that the plateau-insulator transition falls in a non-Fermi
liquid universality class.Comment: 8 pages, accepted for publication in Proceedings of the Yamada
Conference LX on Research in High Magnetic Fields (August 16-19, 2006,
Sendai
Structural and magnetic transition in CeFeAsO: separated or connected?
Using an adapted Sn-flux growth technique we obtained comparatively large
CeFeAsO single crystals of better quality than previously reported polycrystals
or single crystals, as evidenced by much sharper anomalies at the structural
and magnetic phase transitions as well as a much higher residual resistivity
ratio of 12. In the magnetically ordered phase we observe a very pronounced
metallic behavior of the in-plane resistivity, which excludes a Mott insulator
regime at low temperature. The separation Delta_T = T_0 - T_N between
structural and magnetic ordering temperatures decreases with increasing sample
quality, from 18 K in the initial reports to 6 K in the present single
crystals, demonstrating that this separation is not an intrinsic property of
the RFeAsO systems. Our results indicate that the coupling between magnetic
ordering and structural distortion is very similar in AFe2As2 and RFeAsO type
of compounds, much more similar than previously thought. The implications of
our experimental results give arguments both in favor and against the nematic
phase model.Comment: published in PRB with the title 'Coupling between the structural and
magnetic transition in CeFeAsO
Field-induced length changes in the spin-liquid candidate -(BEDT-TTF)Cu(CN)
Measurements of the coefficient of thermal expansion on the spin-liquid
candidate -(BEDT-TTF)Cu(CN) have revealed distinct and
strongly anisotropic lattice effects around 6 K - a possible spin-liquid
instability. In order to study the effects of a magnetic field on the
low-temperature spin-liquid state, dilatometric measurements have been
conducted both as a function of temperature at \emph{B} = const. and as a
function of field at \emph{T} = const. While the 6 K anomaly is found to be
insensitive to magnetic fields \emph{B} 10 T, the maximum field applied,
surprisingly strong \emph{B}-induced effects are observed for magnetic fields
applied along the in-plane \emph{b}-axis. Above a threshold field of 0.5 T <
\emph{B} 1 T, a jump-like anomaly is observed in the \emph{b}-axis
lattice parameter. This anomaly, which is located at 8.7 K at \emph{B} = 1 T,
grows in size and shifts to lower temperatures with increasing the magnetic
field. Although the anomaly bears resemblance to a first-order phase
transition, the lack of hysteresis suggests otherwise.Comment: 3 pages, 3 figures, proceedings of ISCOM 2011, physica status solidi
(c)(in press
New Insights into the Plateau-Insulator Transition in the Quantum Hall Regime
We have measured the quantum critical behavior of the plateau-insulator (PI)
transition in a low-mobility InGaAs/GaAs quantum well. The longitudinal
resistivity measured for two different values of the electron density follows
an exponential law, from which we extract critical exponents kappa = 0.54 and
0.58, in good agreement with the value (kappa = 0.57) previously obtained for
an InGaAs/InP heterostructure. This provides evidence for a non-Fermi liquid
critical exponent. By reversing the direction of the magnetic field we find
that the averaged Hall resistance remains quantized at the plateau value h/e^2
through the PI transition. From the deviations of the Hall resistance from the
quantized value, we obtain the corrections to scaling.Comment: accepted proceedings of EP2DS-15 (to be published in Physica E
Temperature- and quantum phonon effects on Holstein-Hubbard bipolarons
The one-dimensional Holstein-Hubbard model with two electrons of opposite
spin is studied using an extension of a recently developed quantum Monte Carlo
method, and a very simple yet rewarding variational approach, both based on a
canonically transformed Hamiltonian. The quantum Monte Carlo method yields very
accurate results in the regime of small but finite phonon frequencies,
characteristic of many strongly correlated materials such as, e.g., the
cuprates and the manganites. The influence of electron-electron repulsion,
phonon frequency and temperature on the bipolaron state is investigated.
Thermal dissociation of the intersite bipolaron is observed at high
temperatures, and its relation to an existing theory of the manganites is
discussed.Comment: 12 pages, 7 figures; final version, accepted for publication in Phys.
Rev.
Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)X
High-resolution thermal expansion measurements have been performed for
exploring the mysterious "structureless transition" in (TMTTF)X (X =
PF and AsF), where charge ordering at coincides with the
onset of ferroelectric order. Particularly distinct lattice effects are found
at in the uniaxial expansivity along the interstack
-direction. We propose a scheme involving a charge
modulation along the TMTTF stacks and its coupling to displacements of the
counteranions X. These anion shifts, which lift the inversion symmetry
enabling ferroelectric order to develop, determine the 3D charge pattern
without ambiguity. Evidence is found for another anomaly for both materials at
0.6 indicative of a phase transition
related to the charge ordering
Quantum criticality, particle-hole symmetry, and duality of the plateau-insulator transition in the quantum Hall regime
We report new experimental data on the plateau-insulator transition in the
quantum Hall regime, taken from a low mobility InGaAs/InP heterostructure. By
employing the fundamental symmetries of the quantum transport problem we are
able to disentangle the universal quantum critical aspects of the
magnetoresistance data (critical indices and scaling functions) and the sample
dependent aspects due to macroscopic inhomogeneities. Our new results and
methodology indicate that the previously established experimental value for the
critical index (kappa = 0.42) resulted from an admixture of both universal and
sample dependent behavior. A novel, non-Fermi liquid value is found (kappa =
0.57) along with the leading corrections to scaling. The statement of
self-duality under the Chern Simons flux attachment transformation is verified.Comment: 4 pages, 2 figure
Low-temperature lattice effects in the spin-liquid candidate -(BEDT-TTF)Cu(CN)
The quasi-two-dimensional organic charge-transfer salt
-(BEDT-TTF)Cu(CN) is one of the prime candidates for a
quantum spin-liquid due the strong spin frustration of its anisotropic
triangular lattice in combination with its proximity to the Mott transition.
Despite intensive investigations of the material's low-temperature properties,
several important questions remain to be answered. Particularly puzzling are
the 6\,K anomaly and the enigmatic effects observed in magnetic fields. Here we
report on low-temperature measurements of lattice effects which were shown to
be particularly strongly pronounced in this material (R. S. Manna \emph{et
al.}, Phys. Rev. Lett. \textbf{104}, 016403 (2010)). A special focus of our
study lies on sample-to-sample variations of these effects and their
implications on the interpretation of experimental data. By investigating
overall nine single crystals from two different batches, we can state that
there are considerable differences in the size of the second-order phase
transition anomaly around 6\,K, varying within a factor of 3. In addition, we
find field-induced anomalies giving rise to pronounced features in the sample
length for two out of these nine crystals for temperatures 9 K. We
tentatively assign the latter effects to -induced magnetic clusters
suspected to nucleate around crystal imperfections. These -induced effects
are absent for the crystals where the 6\,K anomaly is most strongly pronounced.
The large lattice effects observed at 6\,K are consistent with proposed pairing
instabilities of fermionic excitations breaking the lattice symmetry. The
strong sample-to-sample variation in the size of the phase transition anomaly
suggests that the conversion of the fermions to bosons at the instability is
only partial and to some extent influenced by not yet identified
sample-specific parameters
Performance of Hamamatsu 64-anode photomultipliers for use with wavelength--shifting optical fibres
Hamamatsu R5900-00-M64 and R7600-00-M64 photomultiplier tubes will be used
with wavelength--shifting optical fibres to read out scintillator strips in the
MINOS near detector. We report on measurements of the gain, efficiency,
linearity, crosstalk, and dark noise of 232 of these PMTs, of which 219 met
MINOS requirements.Comment: 15 pages, 12 figures. Accepted by Nucl. Inst. Meth.
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