975 research outputs found
Polaron Effects on Superexchange Interaction: Isotope Shifts of , , and in Layered Copper Oxides
A compact expression has been obtained for the superexchange coupling of
magnetic ions via intermediate anions with regard to polaron effects at both
magnetic ions and intermediate anions. This expression is used to analyze the
main features of the behavior of isotope shifts for temperatures of three types
in layered cuprates: the Neel temperatures (), critical temperatures of
transitions to a superconducting state (), and characteristic temperatures
of the pseudogap in the normal state ().Comment: 4 pages, 1 figur
Superposition models and the multiplicity fluctuations in heavy ion collisions
A class of simple superposition models based on the Glauber picture of
multiple collisions is compared with the data on the centrality dependence of
the multiplicity distributions in a central rapidity bin. We show how the
results depend on the specific assumptions concerning the distributions in the
number of participants and their relations to the distributions of the number
of produced hadrons in various phase space bins. None of the versions of the
model describes satisfactorily the centrality dependence of the scaled
dispersion.Comment: 9 pages, 5 figures, a misprint in formula corrected, accepted for
publication in EPJ
Electronic theory for superconductivity in SrRuO: triplet pairing due to spin-fluctuation exchange
Using a two-dimensional Hubbard Hamiltonian for the three electronic bands
crossing the Fermi level in SrRuO we calculate the band structure and
spin susceptibility in quantitative agreement with
nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS)
experiments. The susceptibility has two peaks at {\bf Q}
due to the nesting Fermi surface properties and at {\bf q}
due to the tendency towards ferromagnetism. Applying spin-fluctuation exchange
theory as in layered cuprates we determine from ,
electronic dispersions, and Fermi surface topology that superconductivity in
SrRuO consists of triplet pairing. Combining the Fermi surface topology
and the results for we can exclude and wave
symmetry for the superconducting order parameter. Furthermore, within our
analysis and approximations we find that -wave symmetry is slightly favored
over p-wave symmetry due to the nesting properties of the Fermi surface.Comment: 5 pages, 5 figures, misprints correcte
Spin dynamics in the low-dimensional magnet TiOCl
We present detailed ESR investigations on single crystals of the
low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates
a stable orbital configuration below room temperature, and allows to estimate
the energy of the first excited state as 0.3(1) eV ruling out a possible
degeneracy of the orbital ground state. Moreover, we discuss the possible spin
relaxation mechanisms in TiOCl and analyze the angular and temperature
dependence of the linewidth up to 250 K in terms of anisotropic exchange
interactions. Towards higher temperatures an exponential increase of the
linewidth is observed, indicating an additional relaxation mechanism.Comment: 10 pages, 8 figures; accepted for publication in Phys. Rev.
Probing the Yb spin relaxation in YYbBaCuO by Electron Paramagnetic Resonance
The relaxation of Yb in YBaCuO () was studied
using Electron Paramagnetic Resonance (EPR). It was found that both electronic
and phononic processes contribute to the Yb relaxation. The phononic
part of the relaxation has an exponential temperature dependence, which can be
explained by a Raman process via the coupling to high-energy (500 K)
optical phonons or an Orbach-like process via the excited vibronic levels of
the Cu ions (localized Slonczewski-modes). In a sample with a maximum
oxygen doping =6.98, the electronic part of the relaxation follows a
Korringa law in the normal state and strongly decreases below .
Comparison of the samples with and without Zn doping proved that the
superconducting gap opening is responsible for the sharp decrease of Yb
relaxation in YBaCuO. It was shown that the electronic
part of the Yb relaxation in the superconducting state follows the same
temperature dependence as Cu and O nuclear relaxations despite
the huge difference between the corresponding electronic and nuclear relaxation
rates.Comment: 8 pages, 6 figure
Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs
The recent discovery of pressure induced superconductivity in the binary
helimagnet CrAs has attracted much attention. How superconductivity emerges
from the magnetic state and what is the mechanism of the superconducting
pairing are two important issues which need to be resolved. In the present
work, the suppression of magnetism and the occurrence of superconductivity in
CrAs as a function of pressure () were studied by means of muon spin
rotation. The magnetism remains bulk up to ~kbar while its volume
fraction gradually decreases with increasing pressure until it vanishes at
7~kbar. At 3.5 kbar superconductivity abruptly appears with its
maximum ~K which decreases upon increasing the pressure. In the
intermediate pressure region (~kbar) the
superconducting and the magnetic volume fractions are spatially phase separated
and compete for phase volume. Our results indicate that the less conductive
magnetic phase provides additional carriers (doping) to the superconducting
parts of the CrAs sample thus leading to an increase of the transition
temperature () and of the superfluid density (). A scaling of
with as well as the phase separation between magnetism and
superconductivity point to a conventional mechanism of the Cooper-pairing in
CrAs.Comment: 9 pages, 8 figure
Sub-gap optical response across the structural phase transition in van der Waals layered \alpha-RuCl
We report magnetic, thermodynamic, thermal expansion, and on detailed optical
experiments on the layered compound -RuCl focusing on the THz and
sub-gap optical response across the structural phase transition from the
monoclinic high-temperature to the rhombohedral low-temperature structure,
where the stacking sequence of the molecular layers is changed. This type of
phase transition is characteristic for a variety of tri-halides crystallizing
in a layered honeycomb-type structure and so far is unique, as the
low-temperature phase exhibits the higher symmetry. One motivation is to
unravel the microscopic nature of spin-orbital excitations via a study of
temperature and symmetry-induced changes. We document a number of highly
unusual findings: A characteristic two-step hysteresis of the structural phase
transition, accompanied by a dramatic change of the reflectivity. An electronic
excitation, which appears in a narrow temperature range just across the
structural phase transition, and a complex dielectric loss spectrum in the THz
regime, which could indicate remnants of Kitaev physics. Despite significant
symmetry changes across the monoclinic to rhombohedral phase transition, phonon
eigenfrequencies and the majority of spin-orbital excitations are not strongly
influenced. Obviously, the symmetry of the single molecular layers determine
the eigenfrequencies of most of these excitations. Finally, from this combined
terahertz, far- and mid-infrared study we try to shed some light on the so far
unsolved low energy (< 1eV) electronic structure of the ruthenium
electrons in -RuCl.Comment: 22 pages, 9 figure
Electronic theory for the normal state spin dynamics in SrRuO: anisotropy due to spin-orbit coupling
Using a three-band Hubbard Hamiltonian we calculate within the
random-phase-approximation the spin susceptibility, , and
NMR spin-lattice relaxation rate, 1/T, in the normal state of the triplet
superconductor SrRuO and obtain quantitative agreement with
experimental data. Most importantly, we find that due to spin-orbit coupling
the out-of-plane component of the spin susceptibility becomes at
low temperatures two times larger than the in-plane one. As a consequence
strong incommensurate antiferromagnetic fluctuations of the
quasi-one-dimensional - and -bands point into the -direction. Our
results provide further evidence for the importance of spin fluctuations for
triplet superconductivity in SrRuO.Comment: revised versio
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