arXiv subjects

T. Schneider

Publications and source records attributed to T. Schneider.

At least 91 records · Page 5Linked to original sources

arXiv2004

Granular superconductivity in the cuprates evinced by finite size effects in the specific heat and London penetration depths

We review and refine the finite size scaling analysis of specific heat and London penetration depths data of cuprate superconductors and compare it to the analysis of specific heat measurements near the superfluid transition of 4He confined to 1mum^3 cylindrical boxes. This system crosses from 3D to 0D behavior near the transition. This has a marked effect on the specific heat as seen by a pronounced rounding of the maximum and a shift to a temperature lower than the transition temperature of the bulk system. The region in between the 3D to 0D crossover uncovers the contributions from the surface and the edges of the cylindrical boxes. Our finite size scaling analysis of the specific heat and London penetration depth of high quality Y-123 and Bi-2212 single crystals uncover essentially the same crossover phenomena, including evidence for surface and edge contributions. These uncovers granular superconductivity in the samples considered here.

cond-mat.supr-con↗

arXiv2003

Finite size effect in Bi2Sr2CaCu2O(8+d) and YBa2Cu3O6.7 probed by the in-plane and out of plane penetration depths

We report on a systematic finite size scaling analysis of in-plane penetration depth data taken on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ epitaxially-grown films and single crystals, and of in-plane and out of plane data taken on YBa$_{2}$Cu$_{3}$O$_{6.7}$ aligned powder. It is shown that the tails in temperature dependence of the penetration depths, appearing around the transition temperature, are fully consistent with a finite size effect. This uncovers the granular nature of these cuprates, consisting of superconducting homogeneous domains of nanoscale extent, embedded in a non-superconducting matrix.

cond-mat.supr-con↗

arXiv2003

The mystery of superconductivity in the cuprates evinced by London penetration depths measurements

The London penetration depth plays a key role in determining and uncovering many properties of a superconductor, including homogeneity, anisotropy, isotope effects, importance of quantum and thermal fluctuations, and facets of the nature of superconductivity in a particular material. Guided by the generic phase diagram in the temperature-dopant concentration plane we examine experimental data on the temperature, isotope substitution, inhomogeneity and magnetic field dependence of the penetration depths to uncover some facets of the mystery of superconductivity in the cuprates

cond-mat.supr-con↗

arXiv2003

Probing inhomogeneities in type II superconductors by means of thermal fluctuations, magnetic fields and isotope effects

Type II superconductors, consisting of superconducting domains embedded in a normal or insulating matrix, undergo a rounded phase transition. Indeed, the correlation length cannot grow beyond the spatial extent of the domains. Accordingly, the thermodynamic properties will exhibit a finite size effect. It is shown that the specific heat and penetration depth data of a variety of type II superconductors, including cuprates, exhibit the characteristic properties of a finite size effect, arising from domains with nanoscale extent. The finite size scaling analysis reveals essential features of the mechanism. Transition temperature and superfluidity increase with reduced domain size. The combined finite size and isotope effects uncover the relevance of local lattice distortions

cond-mat.supr-con↗

arXiv2003

On the role of shake-off in single-photon double ionization

The role of shake-off for double ionization of atoms by a single photon with finite energy has become the subject of debate. In this letter, we attempt to clarify the meaning of shake-off at low photon energies by comparing different formulations appearing in the literature and by suggesting a working definition. Moreover, we elaborate on the foundation and justification of a mixed quantum-classical ansatz for the calculation of single-photon double ionization.

physics.atom-ph↗

arXiv2003

Quasiclassical double photoionization from the 2^{1,3}S excited states of helium including shakeoff

We account for the different symmetries of the 2^{1,3}S helium excited states in a quasiclassical description of the knockout mechanism augmented by a quantum shakeoff contribution. We are thus able to formulate the separate contribution of the knockout and shakeoff mechanisms for double photoionization for any excess energy from the 2^{1,3}S states. Photoionization ratios and singly differential cross sections calculated for the 2^{1,3}S excited states of helium are found to be in very good agreement with recent theoretical results.

physics.atom-ph↗

arXiv2003

Relevance of electron-lattice coupling in cuprates superconductors

We study the oxygen isotope (^{16}O,^{18}O) and finite size effects in Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7-δ} by in-plane penetration depth (λ_{ab}) measurements. A significant change of the length L_{c} of the superconducting domains along the c-axis and λ_{ab}^{2} is deduced, yielding the relative isotope shift ΔL_{c}/L_{c}\approx Δλ_{ab}^{2}/λ_{ab}^{2}\approx -0.14 for x=0, 0.2 and 0.3. This uncovers the existence and relevance of the coupling between the superfluid, lattice distortions and anharmonic phonons which involve the oxygen lattice degrees of freedom.

cond-mat.supr-con↗

arXiv2003

Thermodynamic relevance of nanoscale inhomogeneities in Bi2Sr2CaCu2O8+delta

Using finite size scaling we analyze specific heat and London penetration depth data to estimate the spatial extent of the superconducting grains in so called high-quality Bi2Sr2CaCu2O8+delta single crystals. Contrary to the previously investigated type II superconductors, YBa2Cu3O7+delta, MgB2,2H-NbSe2 and Nb77Zr23, where the lower bound for the length scale of the grains ranges from 182 A to 814 A, our analysis uncovers nanoscale superconducting grains. This clarifies the relevance of the spatial variations in the electronic characteristics observed in underdoped Bi2Sr2CaCu2O8+delta with scanning tunnelling microscopy for bulk and thermodynamic properties and establishes their spatial extent.

cond-mat.supr-con↗

arXiv2002

Probing thermal fluctuations and inhomogeneities in type II superconductors by means of applied magnetic fields

A superconductor is influenced by an applied magnetic field. Close to the transition temperature $T_{c}$ fluctuations dominate and the correlation length $ξ$ increases strongly when $T_{c}$ is approached. However, for nonzero magnetic field $H$ there is another length scale $L_{H}=\sqrt{Φ_{0}/aH}$ where $a$ is a universal amplitude. It is comparable to the average distance between vortex lines. We show that the correlation length is bounded by this length scale, so that $ξ$ cannot grow beyond $L_{H}$. This implies that type II superconductors in a magnetic field do not undergo a phase transition to a state with zero resistance. We sketch the scaling theory of the resulting magnetic field induced finite size effect. In contrast to its inhomogeneity induced counterpart, the magnetic length scale can be varied continuously in terms of the magnetic field strength. This opens the possibility to assess the importance of fluctuations, to extract critical point properties of the homogeneous system and to derive a lower bound for the length scale of inhomogeneities which affect thermodynamic properties. Our analysis of specific heat data for under- and optimally doped YBa$_{2}$Cu$_{3}$O$_{7-δ}$, MgB$_{2}$, 2H-NbSe$_{2}$ and Nb$_{77} $Zr$_{23}$ confirms this expectation. The resulting lower bounds are in the range from 182 A to 818 A. Since the available data does not extend to low fields, much larger values are conceivable. This raises serious doubts on the relevance of the nanoscale spatial variations in the electronic characteristics observed with scanning tunnelling microscopy.

cond-mat.supr-con↗

arXiv2002

Relationship between the isotope effects on transition temperature, specific heat and penetration depths

We show that in anisotropic superconductors, falling at finite temperature into the 3D-XY- and at zero temperature into the 2D-XY-QSI universality class, the isotope effects on transition temperature, specific heat and magnetic penetration depths are not independent, but related by universal relations. The corresponding experimental data for cuprate superconductors reveals suggestive consistency with these relations. They imply a dominant role of fluctuations so that pair formation and pair condensation do not occur simultaneously. For this reason and due to the occurrence of the 2D-XY-QSI transition the isotope effects do not provide information on the underlying pairing mechanism but must be attributed to a shift of the phase diagram upon isotope substitution. The 2D-QSI transition is driven by the exchange of pairs, which favors superconductivity and the combined effects of disorder and Coulomb repulsion of the pairs, which favor localization. Since isotope substitution will hardly affect disorder and Coulomb repulsion, the shift of the phase diagram must be attributed to the electron-phonon interaction, which renormalizes the mass of the fermions and with that the mass and the exchange of the pairs.

cond-mat.supr-con↗

arXiv2002

Universal properties of cuprate superconductors

To provide an understanding of the universal properties emerging from the empirical correlations and phase diagrams of cuprate superconductors, we invoke the scaling theory of finite temperature and quantum critical phenomena. The universal features are traced back to the existence of quantum critical lines, representing the end lines of the finite temperature transition surface. At the respective quantum critical lines 2D superconductor to insulator (2D-QSI) transitions and 3D superconductor to normal state (3D-QSN) transitions occur. The flow to this quantum critical points is tuned by doping, substitution and anisotropy. It is shown that the empirical correlations, like the dependence of Tc on dopant and substitution concentration, the dependence of Tc on zero temperature in-plane penetration depth, etc., reflect universal properties associated with the flow to these quantum critical points and of the crossover from one to the other. A detailed account of the flow to 2D-QSI and 3D-QSN criticality is a challenge for microscopic theories attempting to solve the puzzle of superconductivity in these materials.

cond-mat.supr-con↗

arXiv2002

Competition between anisotropy and superconductivity in organic and cuprate superconductors

We analyze the empirical correlation between the zero temperature penetration depth $λ_{c}(0) $ and the corresponding normal state DC conductivity $σ_{c}^{DC}$, measured slightly above the transition temperature $T_{c}$, in different classes of quasi two-dimensional superconductors, including cuprates and organics. For this purpose we invoke the scaling theory of quantum and finite temperature critical phenomena. Important implications are: Superconductivity in the organic and cuprate superconductors is a genuine three dimensional (3D) phenomenon. The competition between anisotropy and superconductivity destroys the latter in the 2D limit even in the ground state. The data uncovers the flow to quantum criticality, including the 2D quantum superconductor to insulator (2D-QSI) and the 3D quantum superconductor to normal state (3D-QSN) transition. This flow gives a clear perspective of the regimes where quantum fluctuations are essential and mean-field treatments fail. Thus, a detailed account of the flow from mean-field to 2D-QSI criticality in organics and the crossover from the 2D-QSI to the 3D-QSN critical point in cuprates is a challenge for microscopic theories attempting to solve the puzzle of superconductivity in these materials.

cond-mat.supr-con↗

arXiv2002

Universal properties of cuprate superconductors: Evidence and Implications

To provide an understanding of the universal thermodynamic properties of cuprate superconductors, emerging from the empirical correlations and phase diagrams, we analyze them in terms of the scaling theory of finite temperature and quantum critical phenomena. The universal features are traced back to the existence of quantum critical lines, representing the end lines of the finite temperature transition surface. At the respective quantum critical lines two dimensional superconductor to insulator (2D-QSI) transitions and three dimensional superconductor to normal state (3D-QSN) transitions occur. The flow to this quantum critical points is tuned by doping, substitution and anisotropy. It is shown that the empirical correlations, like the dependence of $T_{c}$on dopant and substitution concentration, the dependence of $T_{c}$ on zero temperature in-plane penetration depth, etc., as well as the anomalous isotope effect on $T_{c}$ and penetration depth, reflect the universal properties associated with the flow to these quantum critical points and of the crossover from one to the other.

cond-mat.supr-con↗

arXiv2001

Evidence for three dimensional superconductivity in the cuprates oxides

We review the empirical scenario emerging from the measured doping dependence of the transition temperature and the anisotropy parameter $% γ$ $=ξ_{ab}/ξ_{c}$, defined as the ratio of the correlation lengths parallel and perpendicular to the ab-planes. It suggests that two dimensional models cannot explain the occurrence of superconductivity in the cuprates. This conclusion is confirmed and extended in terms of a novel scaling relation. It involves the transition temperature, the aerial superfluid density in the ground state, $γ$ and the dynamic critical exponent of the quantum superconductor to insulator transition. The important implication there is that a non vanishing superfluid density in the ground state of the cuprates is unalterably linked to an anisotropic but 3D condensation mechanism.

cond-mat.supr-con↗

arXiv2001

Nature of the quantum insulator to superconductor and superconductor to normal state transitions in cuprate superconductors

Using the scaling theory of quantum critical phenomena we present a detailed analysis of various superconducting properties of La$_{2-x}$Sr$_{x}$CuO$_{4}$, Y$_{0.8}$Ca$_{0.2}$-123, Tl-1212 and Tl-2201. Our results include: the experimental data are fully consistent with a 2D insulator to superconductor (ISQ) transition in the underdoped limit. The quantum critical properties appear to be equivalent to those of a dirty 2D bosonic system with long range Coulomb interaction. Together with previous evidence, this transition appears to be a generic feature of cuprate superconductors. The 3D superconductor to normal state quantum transition (SNQ) turns out to be less unique. More extended experimental data is needed to settle the nature of this transition unambiguously.

cond-mat.supr-con↗

arXiv2000

Dynamic critical properties of the vortex-glass transition derived from angular-dependent properties of La$_{2-x}$Sr$_{x}$CuO$_{4}$ films

We present resistivity data on a high-quality La$_{2-x}$Sr$_{x}$CuO$_{4-δ}$ film measured in a magnetic field of 1 T applied at an angle $δ$ to the c axis. Using these data, the influence of the orientation of the magnetic field and the effective mass anisotropy on the vortex-glass transition can be studied. The variation of $δ$ for a fixed magnitude of the magnetic field allows us to investigate the critical properties of interest, including the 2D-to-3D crossover and the 3D vortex glass-to-fluid transition, as the temperature is decreased. The data are well described by the scaling theory for the d.c. resistivity of an anisotropic superconductor in a magnetic field applied at an angle $δ$ to the c axis. This scaling includes the critical properties close to and at the vortex-glass transition. The main results include (i) evidence of a Kosterlitz-Thouless transition in zero field, and (ii) a 2D-to-3D crossover at H=1 T as the temperature is decreased below the zero-field transition temperature, leading to the vortex fluid-to-vortex glass transition in D = 3 characterized by the dynamic critical exponent $z\approx 5.7$.

cond-mat.supr-con↗

arXiv2000

Isotope effects in underdoped cuprate superconductors: a quantum phenomenon

We show that the unusual doping dependence of the isotope effects on transition temperature and zero temperature in - plane penetration depth naturally follows from the doping driven 3D-2D crossover, the 2D quantum superconductor to insulator transition (QSI) in the underdoped limit and the change of the relative doping concentration upon isotope substitution. Close to the QSI transition both, the isotope coefficient of transition temperature and penetration depth approach the coefficient of the relative dopant concentration, and its divergence sets the scale. These predictions are fully consistent with the experimental data and imply that close to the underdoped limit the unusual isotope effect on transition temperature and penetration depth uncovers critical phenomena associated with the quantum superconductor to insulator transition in two dimensions.

cond-mat.supr-con↗

arXiv2000

Lorentz angle measurements in irradiated silicon detectors between 77 K and 300 K

Future experiments are using silicon detectors in a high radiation environment and in high magnetic fields. The radiation tolerance of silicon improves by cooling it to temperatures below 180 K. At low temperatures the mobility increases, which leads to larger deflections of the charge carriers by the Lorentz force. A good knowledge of the Lorentz angle is needed for design and operation of silicon detectors. We present measurements of the Lorentz angle between 77 K and 300 K before and after irradiation with a primary beam of 21 MeV protons.

physics.ins-det↗