1,222 research outputs found
Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?
Up to now, no laboratory-based study has investigated polycyclic aromatic
hydrocarbon (PAH) species as potential carriers of both the diffuse
interstellar bands (DIBs) and the 2175 A UV bump. We examined the proposed
correlation between these two features by applying experimental and theoretical
techniques on two specific medium-sized/large PAHs (dibenzorubicene C30H14 and
hexabenzocoronene C42H18) in their neutral and cationic states. It was already
shown that mixtures of sufficiently large, neutral PAHs can partly or even
completely account for the UV bump. We investigated how the absorption bands
are altered upon ionization of these molecules by interstellar UV photons. The
experimental studies presented here were realized by performing matrix
isolation spectroscopy with subsequent far-UV irradiation. The main effects
were found to be a broadening of the absorption bands in the UV combined with
slight red shifts. The position of the complete pi - pi* absorption structure
around 217.5 nm, however, remains more or less unchanged which could explain
the observed position invariance of the interstellar bump for different lines
of sight. This favors the assignment of this feature to the interstellar PAH
population. As far as the DIBs are concerned, neither our investigations nor
the laboratory studies carried out by other research groups support a possible
connection with this class of molecules. Instead, there are reasonable
arguments that neutral and singly ionized cationic PAHs cannot be made
responsible for the DIBs.Comment: 11 pages, 7 figures, 1 tabl
Ferromagnetic quantum critical fluctuations in YbRh_2(Si_{0.95}Ge_{0.05})_2
The bulk magnetic susceptibility of
YbRh(SiGe) has been investigated %by ac-and
dc-magnetometry at low temperatures and close to the field-induced quantum
critical point at T. For T a Curie-Weiss law with a
negative Weiss temperature is observed at temperatures below 0.3 K. Outside
this region, the susceptibility indicates ferromagnetic quantum critical
fluctuations: above 0.3 K, while at low temperatures
the Pauli susceptibility follows and scales with
the coefficient of the term in the electrical resistivity. The
Sommerfeld-Wilson ratio is highly enhanced and increases up to 30 close to the
critical field.Comment: Physical Review Letters, to be publishe
Anisotropic electron spin resonance of YbIr2Si2
A series of electron spin resonance (ESR) experiments were performed on a
single crystal of the heavy fermion metal YbIr2Si2 to map out the anisotropy of
the ESR-intensity I_ESR which is governed by the microwave field component of
the g-factor. The temperature dependencies of I_ESR(T) and g(T) were measured
for different orientations and compared within the range 2.6K \le T \le 16K.
The analysis of the intensity dependence on the crystal orientation with
respect to both the direction of the microwave field and the static magnetic
field revealed remarkable features: The intensity variation with respect to the
direction of the microwave field was found to be one order of magnitude smaller
than expected from the g-factor anisotropy. Furthermore, we observed a weak
basal plane anisotropy of the ESR parameters which we interpret to be an
intrinsic sample property.Comment: 10 pages, 5 figure
Magnetotransport in the CeIrIn system: The influence of antiferromagnetic fluctuations
We present an overview of magnetotransport measurements on the heavy-fermion
superconductor CeIrIn. Sensitive measurements of the Hall effect and
magnetoresistance (MR) are used to elucidate the low temperature phase diagram
of this system. The normal-state magnetotransport is highly anomalous, and
experimental signatures of a pseudogap-like precursor state to
superconductivity as well as evidence for two distinct scattering times
governing the Hall effect and the MR are observed. Our observations point out
the influence of antiferromagnetic fluctuations on the magnetotransport in this
class of materials. The implications of these findings, both in the context of
unconventional superconductivity in heavy-fermion systems as well as in
relation to the high temperature superconducting cuprates are discussed
Pair breaking by nonmagnetic impurities in the noncentrosymmetric superconductor CePt3Si
We have studied the effect of Ge substitution and pressure on the
heavy-fermion superconductor CePt3Si. Ge substitution on the Si site acts as
negative chemical pressure leading to an increase in the unit-cell volume but
also introduces chemical disorder. We carried out electrical resistivity and ac
heat-capacity experiments under hydrostatic pressure on CePt3Si1-xGex (x=0,
0.06). Our experiments show that the suppression of superconductivity in
CePt3Si1-xGex is mainly caused by the scattering potential, rather than volume
expansion, introduced by the Ge dopants. The antiferromagnetic order is
essentially not affected by the chemical disorder.Comment: 4 pages, 4 figure
Electron Spin Resonance of the Yb 4f moment in Yb(Rh1-xCox)2Si2
[published in Phys. Rev. B 85, 035119 (2012)] The evolution of spin dynamics
from the quantum critical system YbRh2Si2 to the stable trivalent Yb system
YbCo2Si2 was investigated by Electron Spin Resonance (ESR) spectroscopy. While
the Kondo temperature changes by one order of magnitude, all compositions of
the single crystalline series Yb(Rh1-xCox)2Si2 show well defined ESR spectra
with a clear Yb3+ character for temperatures below \approx 20 K. With
increasing Co-content the ESR g-factor along the c-direction strongly increases
indicating a continuous change of the ground state wave function and, thus, a
continuous change of the crystal electric field. The linewidth presents a
complex dependence on the Co-content and is discussed in terms of the Co-doping
dependence of the Kondo interaction, the magnetic anisotropy and the influence
of ferromagnetic correlations between the 4f states. The results provide
evidence that, for low Co-doping, the Kondo interaction allows narrow ESR
spectra despite the presence of a large magnetic anisotropy, whereas at high
Co-concentrations, the linewidth is controlled by ferromagnetic correlations. A
pronounced broadening due to critical correlations at low temperatures is only
observed at the highest Co-content. This might be related to the presence of
incommensurate magnetic fluctuations.Comment: 8 pages, 8 Figure
Electronic spectra of linear HCH and cumulene carbene HC
The transition of linear HCH
(A) has been observed in a neon matrix and gas phase. The assignment is based
on mass-selective experiments, extrapolation of previous results of the longer
HCH homologues, and density functional and multi-state CASPT2
theoretical methods. Another band system starting at 303 nm in neon is assigned
as the transition of the cumulene carbene
pentatetraenylidene HC (B).Comment: 7 pages, 4 figures, 5 table
Hall effect in heavy-fermion metals
The heavy fermion systems present a unique platform in which strong
electronic correlations give rise to a host of novel, and often competing,
electronic and magnetic ground states. Amongst a number of potential
experimental tools at our disposal, measurements of the Hall effect have
emerged as a particularly important one in discerning the nature and evolution
of the Fermi surfaces of these enigmatic metals. In this article, we present a
comprehensive review of Hall effect measurements in the heavy-fermion
materials, and examine the success it has had in contributing to our current
understanding of strongly correlated matter. Particular emphasis is placed on
its utility in the investigation of quantum critical phenomena which are
thought to drive many of the exotic electronic ground states in these systems.
This is achieved by the description of measurements of the Hall effect across
the putative zero-temperature instability in the archetypal heavy-fermion metal
YbRhSi. Using the CeIn (with Co, Ir) family of systems as
a paradigm, the influence of (antiferro-)magnetic fluctuations on the Hall
effect is also illustrated. This is compared to prior Hall effect measurements
in the cuprates and other strongly correlated systems to emphasize on the
generality of the unusual magnetotransport in materials with non-Fermi liquid
behavior.Comment: manuscript accepted in Adv. Phy
Field-induced suppression of the heavy-fermion state in YbRh_2Si_2
We report DC magnetization measurements on YbRh_2Si_2 at temperatures down to
0.04K, magnetic fields B<11.5T and under hydrostatic pressure P<1.3GPa. At
ambient pressure a kink at B*=9.9T indicates a new type of field-induced
transition from an itinerant to a localized 4f-state. This transition is
different from the metamagnetic transition observed in other heavy fermion
compounds, as here ferromagnetic rather than antiferromagnetic correlations
dominate below B*. Hydrostatic pressure experiments reveal a clear
correspondence of B* to the characteristic spin fluctuation temperature
determined from specific heat
- …