51 research outputs found
Controlled vaporization of the superconducting condensate in cuprate superconductors sheds light on the pairing boson
We use ultrashort intense laser pulses to study superconducting state
vaporization dynamics in La(2-x)Sr(x)CuO4 (x=0.1 and 0.15) on the femtosecond
timescale. We find that the energy density required to vaporize the
superconducting state is 2+- 0.8 K/Cu and 2.6 +- 1 K/Cu for x=0.1 and 0.15
respectively. This is significantly greater than the condensation energy
density, indicating that the quasiparticles share a large amount of energy with
the boson glue bath on this timescale. Considering in detail both spin and
lattice energy relaxation pathways which take place on the relevant timescale
of picoseconds, we rule out purely spin-mediated pair-breaking in favor of
phonon-mediated mechanisms, effectively ruling out spin-mediated pairing in
cuprates as a consequence.Comment: 5 pages of article plus 4 pages of supplementary materia
Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals
We investigate the quasiparticle relaxation and low-energy electronic
structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2}
single crystals - exhibiting spin-density wave (SDW) ordering and
superconductivity respectively - using pump-probe femtosecond spectroscopy. In
the undoped single crystals a single relaxation process is observed, showing a
remarkable critical slowing down of the QP relaxation dynamics at the SDW
transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC)
crystals multiple relaxation processes are present, with distinct SC state
quasiparticle recombination dynamics exhibiting a BCS-like T-dependent
superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K
indicating the existence of a pseudogap of magnitude
2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon
energy dependence we conclude that the SC state and PG relaxation channels are
independent, implying the presence of two separate electronic subsystems. We
discuss the data in terms of spatial inhomogeneity and multi-band scenarios,
finding that the latter is more consistent with the present data.Comment: Replaced by the correct versio
Antimicrobial Resistance and Virulence Genes in Enterococcus faecium and Enterococcus faecalis from Humans and Retail Red Meat
The emergence of antimicrobial-resistant and virulent enterococci is a major public health concern. While enterococci are commonly found in food of animal origin, the knowledge on their zoonotic potential is limited. The aim of this study was to determine and compare the antimicrobial susceptibility and virulence traits of Enterococcus faecalis and Enterococcus faecium isolates from human clinical specimens and retail red meat in Slovenia. A total of 242 isolates were investigated: 101 from humans (71 E. faecalis, 30 E. faecium) and 141 from fresh beef and pork (120 E. faecalis, 21 E. faecium). The susceptibility to 12 antimicrobials was tested using a broth microdilution method, and the presence of seven common virulence genes was investigated using PCR. In both species, the distribution of several resistance phenotypes and virulence genes was disparate for isolates of different origin. All isolates were susceptible to daptomycin, linezolid, teicoplanin, and vancomycin. In both species, the susceptibility to antimicrobials was strongly associated with a food origin and the multidrug resistance, observed in 29.6% of E. faecalis and 73.3% E. faecium clinical isolates, with a clinical origin (Fisher's exact test). Among meat isolates, in total 66.0% of E. faecalis and E. faecium isolates were susceptible to all antimicrobials tested and 32.6% were resistant to either one or two antimicrobials. In E. faecalis, several virulence genes were significantly associated with a clinical origin; the most common (31.0%) gene pattern included all the tested genes except hyl. In meat isolates, the virulence genes were detected in E. faecalis only and the most common pattern included ace, efaA, and gelE (32.5%), of which gelE showed a statistically significant association with a clinical origin. These results emphasize the importance of E. faecalis in red meat as a reservoir of virulence genes involved in its persistence and human infections with reported severe outcomes
Tracking Cooper Pairs in a Cuprate Superconductor by Ultrafast Angle-Resolved Photoemission
In high-temperature superconductivity, the process that leads to the
formation of Cooper pairs, the fundamental charge carriers in any
superconductor, remains mysterious. We use a femtosecond laser pump pulse to
perturb superconducting Bi2Sr2CaCu2O8+{\delta}, and study subsequent dynamics
using time- and angle-resolved photoemission and infrared reflectivity probes.
Gap and quasiparticle population dynamics reveal marked dependencies on both
excitation density and crystal momentum. Close to the d-wave nodes, the
superconducting gap is sensitive to the pump intensity and Cooper pairs
recombine slowly. Far from the nodes pumping affects the gap only weakly and
recombination processes are faster. These results demonstrate a new window into
the dynamical processes that govern quasiparticle recombination and gap
formation in cuprates.Comment: 22 pages, 9 figure
Nonlinear order parameter oscillations and lattice coupling in strongly-driven charge-density-wave systems
The anharmonic response of charge-density wave (CDW) order to strong
laser-pulse perturbations in 1T-TaS_2 and TbTe_3 is investigated by means of a
multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe
remarkable anharmonic effects hitherto undetected in the systems exhibiting
collective charge ordering. The efficiency for additional excitation of the
amplitude mode by a laser pulse becomes periodically modulated after the mode
is strongly excited into a coherently oscillating state. A similar effect is
observed also for some other phonons, where the cross-modulation at the
amplitude-mode frequency indicates anharmonic interaction of those phonons with
the amplitude mode. By analyzing the observed phenomena in the framework of
time-dependent Ginzburg-Landau theory we attribute the effects to the
anharmonicity of the mode potentials inherent to the broken symmetry state of
the CDW systems
Diamagnetism of real-space pairs above Tc in hole doped cuprates
The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev.
B 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper
pairing and vortex liquid above the resistive critical temperature. Instead it
is perfectly compatible with the normal state Landau diamagnetism of real-space
composed bosons, which describes the nonlinear magnetization curves in less
anisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in
strongly anisotropic bismuth-based cuprates in the whole range of available
magnetic fields.Comment: 4 pages, 4 figure
Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors
In the transient state of d-wave superconductors, we investigate the temporal
variation of photoinduced changes in the superfluid weight. We derive the
formula that relates the nonlinear response function to the nonequilibrium
distribution function. The latter qunatity is obtained by solving the kinetic
equation with the electron-electron and the electron-phonon interaction
included. By numerical calculations, a nonexponential decay is found at low
temperatures in contrast to the usual exponential decay at high temperatures.
The nonexponential decay originates from the nonmonotonous temporal variation
of the nonequilibrium distribution function at low energies. The main physical
process that causes this behavior is not the recombination of quasiparticles as
previous phenomenological studies suggested, but the absorption of phonons.Comment: 18 pages, 12 figures; to be published in J. Phys. Soc. Jpn. Vol. 80,
No.
Nodal quasiparticle meltdown in ultra-high resolution pump-probe angle-resolved photoemission
High- cuprate superconductors are characterized by a strong
momentum-dependent anisotropy between the low energy excitations along the
Brillouin zone diagonal (nodal direction) and those along the Brillouin zone
face (antinodal direction). Most obvious is the d-wave superconducting gap,
with the largest magnitude found in the antinodal direction and no gap in the
nodal direction. Additionally, while antinodal quasiparticle excitations appear
only below , superconductivity is thought to be indifferent to nodal
excitations as they are regarded robust and insensitive to . Here we
reveal an unexpected tie between nodal quasiparticles and superconductivity
using high resolution time- and angle-resolved photoemission on optimally doped
BiSrCaCuO. We observe a suppression of the nodal
quasiparticle spectral weight following pump laser excitation and measure its
recovery dynamics. This suppression is dramatically enhanced in the
superconducting state. These results reduce the nodal-antinodal dichotomy and
challenge the conventional view of nodal excitation neutrality in
superconductivity.Comment: 7 pages, 3 figure. To be published in Nature Physic
Froehlich Polaron and Bipolaron: Recent Developments
It is remarkable how the Froehlich polaron, one of the simplest examples of a
Quantum Field Theoretical problem, as it basically consists of a single fermion
interacting with a scalar Bose field of ion displacements, has resisted full
analytical or numerical solution at all coupling since 1950, when its
Hamiltonian was first written. The field has been a testing ground for
analytical, semi-analytical, and numerical techniques, such as path integrals,
strong-coupling perturbation expansion, advanced variational, exact
diagonalisation (ED), and quantum Monte Carlo (QMC) techniques. This article
reviews recent developments in the field of continuum and discrete (lattice)
Froehlich (bi)polarons starting with the basics and covering a number of active
directions of research.Comment: 131 pages, 17 figures, 409 references, appear in Reports on Progress
in Physic
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