81,958 research outputs found
Survival of the d-wave superconducting state near the edge of antiferromagnetism in the cuprate phase diagram
In the cuprate superconductor , hole doping in the
layers is controlled by both oxygen content and the degree of oxygen-ordering.
At the composition , the ordering can occur at room
temperature, thereby tuning the hole doping so that the superconducting
critical temperature gradually rises from zero to 20 K. Here we exploit this to
study the c-axis penetration depth as a function of temperature and doping. The
temperature dependence shows the d-wave superconductor surviving to very low
doping, with no sign of another ordered phase interfering with the nodal
quasiparticles. The only apparent doping dependence is a smooth decline of
superfluid density as Tc decreases.Comment: 4 pages, 3 figure
Plane dimpling and Cu 4s hybridization in YBa_2Cu_3O_x
Oxygen doping dimples the CuO_2 planes of YBa_2Cu_3O_{6.8-6.92} by displacing
copper normal to the planes and further towards Ba. The correlated oxygen
displacements, however, are constrained to the in-plane axes. This displacement
pattern is discussed in terms of doping dependent Cu 4s - 3d hybridizations.Comment: 2 pages, submitted to Physica B (Proc. LT22
Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures
Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x
mesa structures leads to persistent doping of the mesas. Here we employ this
effect for analysis of the doping dependence of the electronic spectra of
Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are
able to controllably and reversibly change the doping state of the same single
crystal from underdoped to overdoped state, without changing its chemical
composition. It is observed that such physical doping is affecting
superconductivity in Bi-2212 similar to chemical doping by oxygen impurities:
with overdoping the critical temperature and the superconducting gap decrease,
with underdoping the c-axis critical current rapidly decreases due to
progressively more incoherent interlayer tunneling and the pseudogap rapidly
increases, indicative for the presence of the critical doping point. We
distinguish two main mechanisms of persistent electric doping: (i) even in
voltage contribution, attributed to a charge transfer effect, and (ii) odd in
voltage contribution, attributed to reordering of oxygen impurities
Possible origin of 60-K plateau in the YBa2Cu3O(6+y) phase diagram
We study a model of YBa2Cu3O(6+y) to investigate the influence of oxygen
ordering and doping imbalance on the critical temperature Tc(y) and to
elucidate a possible origin of well-known feature of YBCO phase diagram: the
60-K plateau. Focusing on "phase only" description of the high-temperature
superconducting system in terms of collective variables we utilize a
three-dimensional semi microscopic XY model with two-component vectors that
involve phase variables and adjustable parameters representing microscopic
phase stiffnesses. The model captures characteristic energy scales present in
YBCO and allows for strong anisotropy within basal planes to simulate oxygen
ordering. Applying spherical closure relation we have solved the phase XY model
with the help of transfer matrix method and calculated Tc for chosen system
parameters. Furthermore, we investigate the influence of oxygen ordering and
doping imbalance on the shape of YBCO phase diagram. We find it unlikely that
oxygen ordering alone can be responsible for the existence of 60-K plateau.
Relying on experimental data unveiling that oxygen doping of YBCO may introduce
significant charge imbalance between CuO2 planes and other sites, we show that
simultaneously the former are underdoped, while the latter -- strongly
overdoped almost in the whole region of oxygen doping in which YBCO is
superconducting. As a result, while oxygen content is increased, this provides
two counter acting factors, which possibly lead to rise of 60K plateau.
Additionally, our result can provide an important contribution to understanding
of experimental data supporting existence of multicomponent superconductivity
in YBCO.Comment: 9 pages, 8 figures, submitted to PRB, see http://prb.aps.or
Preparation and Characterization of Homogeneous YBCO Single Crystals with Doping Level near the SC-AFM Boundary
High-purity and homogeneous YBa2Cu3Oy single crystals with carrier doping
level near the AFM-SC boundary have been obtained in the oxygen content range
between y = 6.340 and 6.370. The crystals are ortho-II phase at room
temperature and undergo the orthorhombic to tetragonal transition at about
140_Degree_C. They show sharp superconducting transitions, with Tc between 4
and 20 K. Tc changes by 0.8 K when the oxygen content y is changed by 0.001,
and is also sensitive to annealing conditions near room temperature, due to the
dependence of doping on oxygen ordering correlation lengths. Crystals with
oxygen content y lower than 6.345 are non-superconducting.Comment: 6 page
Theory of Doping and Defects in III-V Nitrides
Doping problems in GaN and in AlGaN alloys are addressed on the basis of
state-of-the-art first-principles calculations. For n-type doping we find that
nitrogen vacancies are too high in energy to be incorporated during growth, but
silicon and oxygen readily form donors. The properties of oxygen, including
DX-center formation, support it as the main cause of unintentional n-type
conductivity. For p-type doping we find that the solubility of Mg is the main
factor limiting the hole concentration in GaN. We discuss the beneficial
effects of hydrogen during acceptor doping. Compensation of acceptors by
nitrogen vacancies may occur, becoming increasingly severe as x increases in
Al_x Ga_(1-x)N alloys.Comment: 7 pages, 2 figures. Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
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