1,026 research outputs found
Thin n-in-p planar pixel sensors and active edge sensors for the ATLAS upgrade at HL-LHC
Silicon pixel modules employing n-in-p planar sensors with an active
thickness of 200 m, produced at CiS, and 100-200 m thin active/slim
edge sensor devices, produced at VTT in Finland have been interconnected to
ATLAS FE-I3 and FE-I4 read-out chips. The thin sensors are designed for high
energy physics collider experiments to ensure radiation hardness at high
fluences. Moreover, the active edge technology of the VTT production maximizes
the sensitive region of the assembly, allowing for a reduced overlap of the
modules in the pixel layer close to the beam pipe. The CiS production includes
also four chip sensors according to the module geometry planned for the outer
layers of the upgraded ATLAS pixel detector to be operated at the HL-LHC. The
modules have been characterized using radioactive sources in the laboratory and
with high precision measurements at beam tests to investigate the hit
efficiency and charge collection properties at different bias voltages and
particle incidence angles. The performance of the different sensor thicknesses
and edge designs are compared before and after irradiation up to a fluence of
.Comment: In proceedings of the 10th International Conference on Position
Sensitive Detectors, PSD10 201
Fermi-surface collapse and dynamical scaling near a quantum critical point
Quantum criticality arises when a macroscopic phase of matter undergoes a
continuous transformation at zero temperature. While the collective
fluctuations at quantum-critical points are being increasingly recognized as
playing an important role in a wide range of quantum materials, the nature of
the underlying quantum-critical excitations remains poorly understood. Here we
report in-depth measurements of the Hall effect in the heavy-fermion metal
YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid
crossover of the isothermal Hall coefficient clearly connected to the
quantum-critical point from a smooth background contribution; the latter exists
away from the quantum-critical point and is detectable through our studies only
over a wide range of magnetic field. Importantly, the width of the critical
crossover is proportional to temperature, which violates the predictions of
conventional theory and is instead consistent with an energy over temperature,
E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our
results provide evidence that the quantum-dynamical scaling and a critical
Kondo breakdown simultaneously operate in the same material. Correspondingly,
we infer that macroscopic scale-invariant fluctuations emerge from the
microscopic many-body excitations associated with a collapsing Fermi-surface.
This insight is expected to be relevant to the unconventional
finite-temperature behavior in a broad range of strongly correlated quantum
systems.Comment: 5 pages, plus supporting materia
Field-Dependent Hall Effect in Single Crystal Heavy Fermion YbAgGe below 1K
We report the results of a low temperature (T >= 50 mK) and high field (H <=
180 kOe) study of the Hall resistivity in single crystals of YbAgGe, a heavy
fermion compound that demonstrates field-induced non-Fermi-liquid behavior near
its field-induced quantum critical point. Distinct features in the anisotropic,
field-dependent Hall resistivity sharpen on cooling down and at the base
temperature are close to the respective critical fields for the field-induced
quantum critical point. The field range of the non-Fermi-liquid region
decreases on cooling but remains finite at the base temperature with no
indication of its conversion to a point for T -> 0. At the base temperature,
the functional form of the field-dependent Hall coefficient is field direction
dependent and complex beyond existing simple models thus reflecting the
multi-component Fermi surface of the material and its non-trivial modification
at the quantum critical point
Simultaneously optimizing the interdependent thermoelectric parameters in Ce(NiCu)Al
Substitution of Cu for Ni in the Kondo lattice system CeNiAl results
in a simultaneous optimization of the three interdependent thermoelectric
parameters: thermoelectric power, electrical and thermal conductivities, where
the electronic change in conduction band induced by the extra electron of Cu is
shown to be crucial. The obtained thermoelectric figure of merit amounts
to 0.125 at around 100 K, comparable to the best values known for Kondo
compounds. The realization of ideal thermoelectric optimization in
Ce(NiCu)Al indicates that proper electronic tuning of Kondo
compounds is a promising approach to efficient thermoelectric materials for
cryogenic application.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Comment on "Zeeman-Driven Lifshitz Transition: A Model for the Experimentally Observed Fermi-Surface Reconstruction in YbRh2Si2"
In Phys. Rev. Lett. 106, 137002 (2011), A. Hackl and M. Vojta have proposed
to explain the quantum critical behavior of YbRh2Si2 in terms of a
Zeeman-induced Lifshitz transition of an electronic band whose width is about 6
orders of magnitude smaller than that of conventional metals. Here, we note
that the ultra-narrowness of the proposed band, as well as the proposed
scenario per se, lead to properties which are qualitatively inconsistent with
the salient features observed in YbRh2Si2 near its quantum critical point.Comment: 3 page
Magnetic field-induced quantum critical point in YbPtIn and YbPtIn single crystals
Detailed anisotropic (Hab and Hc) resistivity and
specific heat measurements were performed on online-grown YbPtIn and
solution-grown YbPtIn single crystals for temperatures down to 0.4 K,
and fields up to 140 kG; Hab Hall resistivity was also measured on
the YbPtIn system for the same temperature and field ranges. All these
measurements indicate that the small change in stoichiometry between the two
compounds drastically affects their ordering temperatures (T
K in YbPtIn, and K in YbPtIn). Furthermore, a field-induced
quantum critical point is apparent in each of these heavy fermion systems, with
the corresponding critical field values of YbPtIn (H around
35-45 kG and H kG) also reduced compared to the analogous
values for YbPtIn (H kG and H kG
Quantum critical properties of the Bose-Fermi Kondo Model in a large-N limit
Studies of non-Fermi liquid properties in heavy fermions have led to the
current interest in the Bose-Fermi Kondo model. Here we use a dynamical large-N
approach to analyze an SU(N)xSU() generalization of the model. We
establish the existence in this limit of an unstable fixed point when the
bosonic bath has a sub-ohmic spectrum (|\omega|^{1-\epsilon} \sgn \omega,
with ). At the quantum critical point, the Kondo scale vanishes
and the local spin susceptibility (which is finite on the Kondo side for \kappa
<1) diverges. We also find an \omega/T scaling for an extended range (15
decades) of \omega/T. This scaling violates (for ) the
expectation of a naive mapping to certain classical models in an extra
dimension; it reflects the inherent quantum nature of the critical point.Comment: 4 pages; v2: included clarifying discussions on why the omega/T
scaling (for epsilon >=1/2) violates the naive mapping to classical models in
an extra dimension and the implications of this observation about the nature
of the QCP; v3: shortened to conform to the PRL length limi
Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling
Recent theoretical studies of topologically nontrivial electronic states in
Kondo insulators have pointed to the importance of spin-orbit coupling (SOC)
for stabilizing these states. However, systematic experimental studies that
tune the SOC parameter in Kondo insulators remain elusive.
The main reason is that variations of (chemical) pressure or doping strongly
influence the Kondo coupling and the chemical potential --
both essential parameters determining the ground state of the material -- and
thus possible tuning effects have remained unnoticed. Here
we present the successful growth of the substitution series
CeBi(PtPd) () of the archetypal
(noncentrosymmetric) Kondo insulator CeBiPt. The Pt-Pd substitution
is isostructural, isoelectronic, and isosize, and therefore likely to leave
and essentially unchanged. By contrast, the large mass
difference between the element Pt and the element Pd leads to a large
difference in , which thus is the dominating tuning
parameter in the series. Surprisingly, with increasing (decreasing
), we observe a Kondo insulator to semimetal transition,
demonstrating an unprecedented drastic influence of the SOC. The fully
substituted end compound CeBiPd shows thermodynamic signatures of a
recently predicted Weyl-Kondo semimetal.Comment: 6 pages, 5 figures plus Supplemental Materia
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