236 research outputs found
A novel non-Fermi-liquid state in the iron-pnictide FeCrAs
We report transport and thermodynamic properties of stoichiometric single
crystals of the hexagonal iron-pnictide FeCrAs. The in-plane resistivity shows
an unusual "non-metallic" dependence on temperature T, rising continuously with
decreasing T from ~ 800 K to below 100 mK. The c-axis resistivity is similar,
except for a sharp drop upon entry into an antiferromagnetic state at T_N 125
K. Below 10 K the resistivity follows a non-Fermi-liquid power law, rho(T) =
rho_0 - AT^x with x<1, while the specific heat shows Fermi liquid behaviour
with a large Sommerfeld coefficient, gamma ~ 30 mJ/mol K^2. The high
temperature properties are reminiscent of those of the parent compounds of the
new layered iron-pnictide superconductors, however the T -> 0 properties
suggest a new class of non-Fermi liquid.Comment: 6 pages, 4 figure
From soft harmonic phonons to fast relaxational dynamics in CHNHPbBr
The lead-halide perovskites, including CHNHPbBr, are
components in cost effective, highly efficient photovoltaics, where the
interactions of the molecular cations with the inorganic framework are
suggested to influence the electronic and ferroelectric properties.
CHNHPbBr undergoes a series of structural transitions
associated with orientational order of the CHNH (MA) molecular
cation and tilting of the PbBr host framework. We apply high-resolution
neutron scattering to study the soft harmonic phonons associated with these
transitions, and find a strong coupling between the PbBr framework and
the quasistatic CHNH dynamics at low energy transfers. At higher
energy transfers, we observe a PbBr octahedra soft mode driving a
transition at 150 K from bound molecular excitations at low temperatures to
relatively fast relaxational excitations that extend up to 50-100 meV.
We suggest that these temporally overdamped dynamics enables possible indirect
band gap processes in these materials that are related to the enhanced
photovoltaic properties.Comment: (main text - 5 pages, 4 figures; supplementary information - 3 pages,
3 figures
Direct relation between the low-energy spin excitations and superconductivity of overdoped high- superconductors
The dynamic spin susceptibility, , has been measured over the
energy range of meV for overdoped
LaSrCuO. Incommensurate (IC) spin excitations are observed
at 8 K for all superconducting samples for with
peaking at meV. The IC peaks at 6 meV become smaller in
intensity with increasing and, finally, become unobservable for a sample
with which has no bulk superconductivity. The maximum
decreases linearly with (onset) in the overdoped region, implying a direct
cooperative relation between the spin fluctuations and the superconductivity.Comment: 4 pages, 4 figure
Non-Fermi-liquid behavior in Ce(RuFe)Ge: cause and effect
We present inelastic neutron scattering measurements on the intermetallic
compounds Ce(RuFe)Ge (=0.65, 0.76 and 0.87). These
compounds represent samples in a magnetically ordered phase, at a quantum
critical point and in the heavy-fermion phase, respectively. We show that at
high temperatures the three compositions have the identical response of a local
moment system. However, at low temperatures the spin fluctuations in the
critical composition are given by non-Fermi-liquid dynamics, while the spin
fluctuations in the heavy fermion system show a simple exponential decay in
time. In both compositions, the lifetime of the fluctuations is determined
solely by the distance to the quantum critical point. We discuss the
implications of these observations regarding the possible origins of
non-Fermi-liquid behavior in this system.Comment: 4 figures, submitted to PR
Two-Loop Self-Energy Corrections to the Fine-Structure
We investigate two-loop higher-order binding corrections to the fine
structure, which contribute to the spin-dependent part of the Lamb shift. Our
calculation focuses on the so-called ``two-loop self-energy'' involving two
virtual closed photon loops. For bound states, this correction has proven to be
notoriously difficult to evaluate. The calculation of the binding corrections
to the bound-state two-loop self-energy is simplified by a separate treatment
of hard and soft virtual photons. The two photon-energy scales are matched at
the end of the calculation. We explain the significance of the mathematical
methods employed in the calculation in a more general context, and present
results for the fine-structure difference of the two-loop self-energy through
the order of .Comment: 19 pages, LaTeX, 2 figures; J. Phys. A (in press); added analytic
results for two-loop form-factor slopes (by P. Mastrolia and E. Remiddi
Magnetic Ordering in the Spin-Ice Candidate HoRuO
Neutron scattering measurements on the spin-ice candidate material
HoRuO have revealed two magnetic transitions at T 95 K and T
1.4 K to long-range ordered states involving the Ru and Ho sublattices,
respectively. Between these transitions, the Ho moments form
short-ranged ordered spin clusters. The internal field provided by the ordered
S=1 Ru moments disrupts the fragile spin-ice state and drives the
Ho moments to order. We have directly measured a slight shift in the
Ho crystal field levels at 95 K from the Ru ordering.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Letter
Magnetic properties of the overdoped superconductor LaSrCuO with and without Zn impurities
The magnetic properties of the Zn-substituted overdoped high-
superconductor LaSrCuZnO have been studied by
magnetization measurements and neutron scattering. Magnetization measurements
reveal that for Zn-free samples with a Curie term is induced in
the temperature dependence of the magnetic susceptibility implying the
existence of local paramagnetic moments. The induced Curie constant corresponds
to a moment of 0.5 per additional Sr ion that exceeds .
Zn-substitution in the overdoped \lsco also induces a Curie term that
corresponds to 1.2 per Zn ion, simultaneously suppressing .
The relationship between and the magnitude of the Curie term for Zn-free
\lsco with and for Zn-substituted \lsco with are
closely similar. This signifies a general competitive relationship between the
superconductivity and the induced paramagnetic moment. Neutron scattering
measurements show that Zn-substitution in overdoped \lsco anomalously enhances
the inelastic magnetic scattering spectra around the position,
peaking at meV. These facts are discussed on the basis of a
"swiss-cheese" model of Zn-substituted systems as well as a microscopic phase
separation scenario in the overdoped region indicated by muon-spin-relaxation
measurements.Comment: 10 pages, 8 figure
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
- …