684 research outputs found
The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films
Temperature dependences of the magnetic moment have been measured in
YBa_2Cu_3O_{7-\delta} thin films over a wide magnetic field range (5 <= H <=
10^4 Oe). In these films a paramagnetic signal known as the paramagnetic
Meissner effect has been observed. The experimental data in the films, which
have strong pinning and high critical current densities (J_c ~ 2 \times 10^6
A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the
theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559
(1995)]. This finding indicates that the origin of the paramagnetic effect is
ultimately associated with nucleation and inhomogeneous spatial redistribution
of magnetic vortices in a sample which is cooled down in a magnetic field. It
is also shown that the distribution of vortices is extremely sensitive to the
interplay of film properties and the real experimental conditions of the
measurements.Comment: RevTex, 8 figure
Impurity induced resonant state in a pseudogap state of a high temperature superconductor
We predict a resonance impurity state generated by the substitution of one Cu
atom with a nonmagnetic atom, such as Zn, in the pseudogap state of a high-T_c
superconductor. The precise microscopic origin of the pseudogap is not
important for this state to be formed, in particular this resonance will be
present even in the absence of superconducting fluctuations in the normal
state. In the presence of superconducting fluctuations, we predict the
existence of a counterpart impurity peak on a symmetric bias.
The nature of impurity resonance is similar to the previously studied
resonance in the d-wave superconducting state.Comment: 4 pages, 2 figure
Microwave response of single crystal YBa₂Cu₃O₇–δ films as a probe for pairing symmetry
Temperature dependences of the microwave surface impedance, Zs(T), are measured in the
c-axis oriented single-crystal high-Tc superconducting cuprate YBa₂Cu₃O₇₋δ (YBCO) thin films
deposited by the off-axis dc magnetron sputtering onto CeO₂-buffered single-crystal sapphire
substrates (film thickness is d ≈ 150, 300, 480 nm). Measurements are performed by a use of the
coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at a number of
several discrete frequencies within the range of 5–134 GHz. The measurements have revealed
unexpected peculiarities on the Zs(T)-dependences for the most perfect films under study. The
peculiarities appear to be most strongly pronounced on the temperature dependences of the film
surface resistance Rs(T) = Re {Zs(T)}. The most important features of the unusual surface
resistance behavior are: (i) the temperature dependence Rs(T) of YBCO films under study at low
temperatures obeys the exponential law: Rs(T) = Rres + R₀ exp [–∆s/T] with the small energy gap
∆s (∆s 0.5Tc at f = 5 GHz); (ii) the most perfect films reveal a distinct two-peak structure of the
Rs(T) dependence with peaks positioned at 27–30 K and 48–51 K, while such peaks are not
observed in less perfect films. The peaks are mostly pronounced at moderate (e.g. 34 GHz)
frequencies and gradually disappear both at higher and lower frequencies, while their temperature
positions remain unchanged. These features of perfect single-crystalline YBCO films are believed
to reveal their intrinsic electron properties. Taking into account the possibility of a mixed
(s+id)-type pairing symmetry as well as a significant role of extended out-of-plane crystal defects
(such as dislocation lines or twin planes) in Bogolyubov’s quasiparticle scattering within the most
perfect YBCO films, one can suggest a consistent explanation for the anomalies observed in the
Zs(T) behavior
Quantum magneto-oscillations in a two-dimensional Fermi liquid
Quantum magneto-oscillations provide a powerfull tool for quantifying
Fermi-liquid parameters of metals. In particular, the quasiparticle effective
mass and spin susceptibility are extracted from the experiment using the
Lifshitz-Kosevich formula, derived under the assumption that the properties of
the system in a non-zero magnetic field are determined uniquely by the
zero-field Fermi-liquid state. This assumption is valid in 3D but, generally
speaking, erroneous in 2D where the Lifshitz-Kosevich formula may be applied
only if the oscillations are strongly damped by thermal smearing and disorder.
In this work, the effects of interactions and disorder on the amplitude of
magneto-oscillations in 2D are studied. It is found that the effective mass
diverges logarithmically with decreasing temperature signaling a deviation from
the Fermi-liquid behavior. It is also shown that the quasiparticle lifetime due
to inelastic interactions does not enter the oscillation amplitude, although
these interactions do renormalize the effective mass. This result provides a
generalization of the Fowler-Prange theorem formulated originally for the
electron-phonon interaction.Comment: 4 pages, 1 figur
Vortex structure in d-density wave scenario of pseudogap
We investigate the vortex structure assuming the d-density wave scenario of
the pseudogap. We discuss the profiles of the order parameters in the vicinity
of the vortex, effective vortex charge and the local density of states. We find
a pronounced modification of these quantities when compared to a purely
superconducting case. Results have been obtained for a clean system as well as
in the presence of a nonmagnetic impurity. We show that the competition between
superconductivity and the density wave may explain some experimental data
recently obtained for high-temperature superconductors. In particular, we show
that the d-density wave scenario explains the asymmetry of the gap observed in
the vicinity of the vortex core.Comment: 8 pages, 10 figure
Big, Fast Vortices in the d-RVB theory of High Temperature Superconductivity
The effect of proximity to a Mott insulating phase on the superflow
properties of a d-wave superconductor is studied using the slave boson-U(1)
gauge theory model. The model has two limits corresponding to superconductivity
emerging either out of a 'renormalized fermi liquid' or out of a
non-fermi-liquid regime. Three crucial physical parameters are identified: the
size of the vortex \textit{as determined from the supercurrent it induces;} the
coupling of the superflow to the quasiparticles and the 'nondissipative time
derivative' term. As the Mott phase is approached, the core size as defined
from the supercurrent diverges, the coupling between superflow and
quasiparticles vanishes, and the magnitude of the nondissipative time
derivative dramatically increases. The dissipation due to a moving vortex is
found to vary as the third power of the doping. The upper critical field and
the size of the critical regime in which paraconductivity may be observed are
estimated, and found to be controlled by the supercurrent length scale
Linear optics substituting scheme for multi-mode operations
We propose a scheme allowing a conditional implementation of suitably
truncated general single- or multi-mode operators acting on states of traveling
optical signal modes. The scheme solely relies on single-photon and coherent
states and applies beam splitters and zero- and single-photon detections. The
signal flow of the setup resembles that of a multi-mode quantum teleportation
scheme thus allowing the individual signal modes to be spatially separated from
each other. Some examples such as the realization of cross-Kerr nonlinearities,
multi-mode mirrors, and the preparation of multi-photon entangled states are
considered.Comment: 11 pages, 4 eps-figures, using revtex
Metastability and Transient Effects in Vortex Matter Near a Decoupling Transition
We examine metastable and transient effects both above and below the
first-order decoupling line in a 3D simulation of magnetically interacting
pancake vortices. We observe pronounced transient and history effects as well
as supercooling and superheating between the 3D coupled, ordered and 2D
decoupled, disordered phases. In the disordered supercooled state as a function
of DC driving, reordering occurs through the formation of growing moving
channels of the ordered phase. No channels form in the superheated region;
instead the ordered state is homogeneously destroyed. When a sequence of
current pulses is applied we observe memory effects. We find a ramp rate
dependence of the V(I) curves on both sides of the decoupling transition. The
critical current that we obtain depends on how the system is prepared.Comment: 10 pages, 15 postscript figures, version to appear in PR
Search for the glueball candidates f0(1500) and fJ(1710) in gamma gamma collisions
Data taken with the ALEPH detector at LEP1 have been used to search for gamma
gamma production of the glueball candidates f0(1500) and fJ(1710) via their
decay to pi+pi-. No signal is observed and upper limits to the product of gamma
gamma width and pi+pi- branching ratio of the f0(1500) and the fJ(1710) have
been measured to be Gamma_(gamma gamma -> f0(1500)). BR(f0(1500)->pi+pi-) <
0.31 keV and Gamma_(gamma gamma -> fJ(1710)). BR(fJ(1710)->pi+pi-) < 0.55 keV
at 95% confidence level.Comment: 10 pages, 3 figure
Search for supersymmetry with a dominant R-parity violating LQDbar couplings in e+e- collisions at centre-of-mass energies of 130GeV to 172 GeV
A search for pair-production of supersymmetric particles under the assumption
that R-parity is violated via a dominant LQDbar coupling has been performed
using the data collected by ALEPH at centre-of-mass energies of 130-172 GeV.
The observed candidate events in the data are in agreement with the Standard
Model expectation. This result is translated into lower limits on the masses of
charginos, neutralinos, sleptons, sneutrinos and squarks. For instance, for
m_0=500 GeV/c^2 and tan(beta)=sqrt(2) charginos with masses smaller than 81
GeV/c^2 and neutralinos with masses smaller than 29 GeV/c^2 are excluded at the
95% confidence level for any generation structure of the LQDbar coupling.Comment: 32 pages, 30 figure
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